diff --git a/app/controllers/concerns/course/users_controller_management_concern.rb b/app/controllers/concerns/course/users_controller_management_concern.rb
index f7a3c47ac4f..0210c642a00 100644
--- a/app/controllers/concerns/course/users_controller_management_concern.rb
+++ b/app/controllers/concerns/course/users_controller_management_concern.rb
@@ -123,7 +123,7 @@ def should_update_personalized_timeline
 
   def course_user_params
     @course_user_params ||= params.require(:course_user).permit(
-      :user_id, :name, :timeline_algorithm, :role, :phantom, :reference_timeline_id
+      :user_id, :name, :timeline_algorithm, :role, :phantom, :reference_timeline_id, :external_id
     )
   end
 
diff --git a/app/controllers/course/user_invitations_controller.rb b/app/controllers/course/user_invitations_controller.rb
index 440addfb908..abd143158f0 100644
--- a/app/controllers/course/user_invitations_controller.rb
+++ b/app/controllers/course/user_invitations_controller.rb
@@ -19,7 +19,8 @@ def create
       create_invitation_success(result)
     else
       propagate_errors
-      render json: { errors: current_course.errors.full_messages.to_sentence }, status: :bad_request
+      errors = current_course.errors[:base]
+      render json: { errors: errors }, status: :bad_request
     end
   end
 
@@ -59,7 +60,8 @@ def course_user_invitation_params
     @course_user_invitation_params ||= begin
       params[:course] = { invitations_attributes: {} } unless params.key?(:course)
       params.require(:course).permit(:invitations_file, :registration_key,
-                                     invitations_attributes: [:name, :email, :role, :phantom, :timeline_algorithm])
+                                     invitations_attributes: [:name, :email, :role, :phantom,
+                                                              :timeline_algorithm, :external_id])
     end
   end
 
@@ -120,7 +122,7 @@ def invite_by_file?
   def invite
     invitation_service.invite(invitation_params)
   rescue CSV::MalformedCSVError => e
-    current_course.errors.add(:invitations_file, e.message)
+    current_course.errors.add(:base, e.message)
     false
   end
 
@@ -135,15 +137,7 @@ def invitation_service
   #
   # @return [void]
   def propagate_errors
-    propagate_errors_to_file if invite_by_file?
-  end
-
-  # Propagates errors from the generated records to the file.
-  #
-  # @return [void]
-  def propagate_errors_to_file
-    errors = aggregate_errors
-    current_course.errors.add(:invitations_file, errors.to_sentence) unless errors.empty?
+    aggregate_errors.each { |msg| current_course.errors.add(:base, msg) }
   end
 
   # Aggregates errors from all the known sources of failure.
@@ -157,9 +151,23 @@ def aggregate_errors
   #
   # @return [Array<String>]
   def invalid_course_user_errors
-    invalid_course_users.map do |course_user|
-      user = self.class.helpers.display_course_user(course_user)
-      t('errors.course.user_invitations.duplicate_user', user: user)
+    invalid_course_users.flat_map do |course_user|
+      email = course_user.user&.email || course_user.name
+      errors = []
+      if course_user.errors[:external_id].any?
+        errors << t('errors.course.user_invitations.duplicate_external_id',
+                    email: email, external_id: course_user.external_id)
+      end
+      if course_user.errors[:user].any? || course_user.errors[:course].any?
+        errors << t('errors.course.user_invitations.already_enrolled', email: email)
+      end
+      other_errors = course_user.errors.reject { |e| %w[external_id user course].include?(e.attribute.to_s) }
+      if other_errors.any?
+        message = other_errors.map { |e| course_user.errors.full_message(e.attribute, e.message) }.
+                  to_sentence
+        errors << t('errors.course.user_invitations.other_error', email: email, message: message)
+      end
+      errors
     end
   end
 
@@ -174,9 +182,25 @@ def invalid_course_users
   #
   # @return [Array<String>]
   def invalid_invitation_email_errors
-    invalid_invitations.map do |invitation|
-      message = invitation.errors.full_messages.to_sentence
-      t('errors.course.user_invitations.invalid_email', email: invitation.email, message: message)
+    invalid_invitations.flat_map do |invitation|
+      email = invitation.email
+      errors = []
+      if invitation.errors[:external_id].any?
+        errors << t('errors.course.user_invitations.duplicate_external_id',
+                    email: email, external_id: invitation.external_id)
+      end
+      if invitation.errors[:email].any?
+        message = invitation.errors[:email].to_sentence
+        errors << t('errors.course.user_invitations.invalid_email', email: email, message: message)
+      end
+      errors << t('errors.course.user_invitations.duplicate_invitation', email: email) if invitation.errors[:base].any?
+      other_errors = invitation.errors.reject { |e| %w[external_id email base].include?(e.attribute.to_s) }
+      if other_errors.any?
+        message = other_errors.map { |e| invitation.errors.full_message(e.attribute, e.message) }.
+                  to_sentence
+        errors << t('errors.course.user_invitations.other_error', email: email, message: message)
+      end
+      errors
     end
   end
 
@@ -254,7 +278,7 @@ def destroy_invitation_success
 
   def destroy_invitation_failure
     respond_to do |format|
-      format.json { render json: { errors: @invitation.errors.full_messages.to_sentence }, status: :bad_request }
+      format.json { render json: { errors: @invitation.errors.full_messages }, status: :bad_request }
     end
   end
 
diff --git a/app/models/concerns/course/unique_external_id_concern.rb b/app/models/concerns/course/unique_external_id_concern.rb
new file mode 100644
index 00000000000..303c36b83a0
--- /dev/null
+++ b/app/models/concerns/course/unique_external_id_concern.rb
@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+# This concern validates that external IDs are unique within a course,
+# across both course users and pending invitations.
+#
+# Nil and blank external IDs are allowed.
+module Course::UniqueExternalIdConcern
+  extend ActiveSupport::Concern
+
+  included do
+    before_validation :normalize_external_id
+
+    validate :validate_unique_external_id_within_course
+  end
+
+  private
+
+  # Normalizes blank external IDs to nil.
+  #
+  # @return [void]
+  def normalize_external_id
+    self.external_id = nil if external_id.blank?
+  end
+
+  # Validates that the external ID is unique within the course,
+  # across both course users and invitations.
+  #
+  # @return [void]
+  def validate_unique_external_id_within_course
+    return if external_id.blank?
+
+    invitation_exists = Course::UserInvitation.
+                        unconfirmed.
+                        where(course_id: course_id, external_id: external_id).
+                        where.not(id: id).
+                        exists?
+
+    course_user_exists = CourseUser.
+                         where(course_id: course_id, external_id: external_id).
+                         where.not(id: id).
+                         exists?
+
+    return unless invitation_exists || course_user_exists
+
+    errors.add(:external_id, :taken)
+  end
+end
diff --git a/app/models/course/user_invitation.rb b/app/models/course/user_invitation.rb
index bfbede2d9d8..e7dab5a9caa 100644
--- a/app/models/course/user_invitation.rb
+++ b/app/models/course/user_invitation.rb
@@ -4,6 +4,8 @@ class Course::UserInvitation < ApplicationRecord
   after_initialize :set_defaults, if: :new_record?
   before_validation :set_defaults, if: :new_record?
 
+  include Course::UniqueExternalIdConcern
+
   validates :email, format: { with: Devise.email_regexp }, if: :email_changed?
   validates :name, presence: true
   validates :role, presence: true
diff --git a/app/models/course_user.rb b/app/models/course_user.rb
index d4c9bd9bfc4..944fe25e09c 100644
--- a/app/models/course_user.rb
+++ b/app/models/course_user.rb
@@ -3,6 +3,7 @@ class CourseUser < ApplicationRecord
   include CourseUser::StaffConcern
   include CourseUser::LevelProgressConcern
   include CourseUser::TodoConcern
+  include Course::UniqueExternalIdConcern
 
   after_initialize :set_defaults, if: :new_record?
   before_validation :set_defaults, if: :new_record?
diff --git a/app/models/user.rb b/app/models/user.rb
index 9415e285035..85923635741 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -129,6 +129,7 @@ def build_course_user_from_invitation(invitation)
                        phantom: invitation.phantom,
                        timeline_algorithm: invitation.timeline_algorithm ||
                           invitation.course&.default_timeline_algorithm,
+                       external_id: invitation.external_id,
                        creator: self,
                        updater: self)
   end
diff --git a/app/models/user/email.rb b/app/models/user/email.rb
index bcb71d25b0a..ab9920c9bf6 100644
--- a/app/models/user/email.rb
+++ b/app/models/user/email.rb
@@ -35,8 +35,14 @@ def accept_all_pending_invitations
           unconfirmed_invitation.confirm!(confirmer: user)
           next
         end
-        user.build_course_user_from_invitation(unconfirmed_invitation)
-        unconfirmed_invitation.confirm!(confirmer: user) if user.save && user.persisted?
+        # Confirm the invitation before saving the CourseUser so that the
+        # UniqueExternalIdConcern validation doesn't reject the new CourseUser
+        # for sharing an external_id with what is now a confirmed invitation.
+        CourseUser.transaction do
+          unconfirmed_invitation.confirm!(confirmer: user)
+          user.build_course_user_from_invitation(unconfirmed_invitation)
+          raise ActiveRecord::Rollback unless user.save && user.persisted?
+        end
       end
     end
   end
diff --git a/app/services/concerns/course/user_invitation_service/parse_invitation_concern.rb b/app/services/concerns/course/user_invitation_service/parse_invitation_concern.rb
index 0db98b9643e..f13bd79f463 100644
--- a/app/services/concerns/course/user_invitation_service/parse_invitation_concern.rb
+++ b/app/services/concerns/course/user_invitation_service/parse_invitation_concern.rb
@@ -48,11 +48,16 @@ def parse_invitations(users)
   def partition_unique_users(users)
     users.each { |user| user[:email] = user[:email].downcase }
     unique_users = {}
+    seen_external_ids = {}
     duplicate_users = []
     users.each do |user|
+      ext_id = user[:external_id].presence
       if unique_users.key?(user[:email])
-        duplicate_users.push(user)
+        duplicate_users.push(user.merge(reason: :duplicate_email))
+      elsif ext_id && seen_external_ids.key?(ext_id)
+        duplicate_users.push(user.merge(reason: :duplicate_external_id))
       else
+        seen_external_ids[ext_id] = true if ext_id
         unique_users[user[:email]] = user
       end
     end
@@ -86,7 +91,8 @@ def parse_from_form(users)
         email: value[:email],
         role: value[:role],
         phantom: phantom,
-        timeline_algorithm: value[:timeline_algorithm] }
+        timeline_algorithm: value[:timeline_algorithm],
+        external_id: value[:external_id] }
     end
   end
 
@@ -144,11 +150,17 @@ def parse_file_row(row)
     return nil if row[1].blank?
 
     row[0] = row[1] if row[0].blank?
-
     role = parse_file_role(row[2])
     phantom = parse_file_phantom(row[3])
-    timeline_algorithm = parse_file_timeline_algorithm(row[4])
-    { name: row[0], email: row[1], role: role, phantom: phantom, timeline_algorithm: timeline_algorithm }
+    if @current_course.show_personalized_timeline_features?
+      timeline_algorithm = parse_file_timeline_algorithm(row[4])
+      external_id = parse_file_external_id(row[5])
+    else
+      external_id = parse_file_external_id(row[4])
+      timeline_algorithm = parse_file_timeline_algorithm(nil)
+    end
+    { name: row[0], email: row[1], role: role, phantom: phantom,
+      timeline_algorithm: timeline_algorithm, external_id: external_id }
   end
 
   # Parses the role column from the CSV file.
@@ -188,6 +200,17 @@ def parse_file_timeline_algorithm(timeline_algorithm)
     symbol || @current_course.default_timeline_algorithm
   end
 
+  # Parses file value for an invitation's external ID.
+  # Returns nil if value is not specified.
+  #
+  # @param [String|nil] external_id External ID as specified in the CSV file.
+  # @return [String|nil] The external ID string, or nil if blank.
+  def parse_file_external_id(external_id)
+    return nil if external_id.blank?
+
+    external_id
+  end
+
   # Removes the UTF-8 byte order mark (BOM) from the string.
   # The BOM exists at the start of in CSVs (optionally) to indicate the
   # encoding of the file.
diff --git a/app/services/concerns/course/user_invitation_service/process_invitation_concern.rb b/app/services/concerns/course/user_invitation_service/process_invitation_concern.rb
index 03c433f79d2..e37076efd81 100644
--- a/app/services/concerns/course/user_invitation_service/process_invitation_concern.rb
+++ b/app/services/concerns/course/user_invitation_service/process_invitation_concern.rb
@@ -73,6 +73,7 @@ def add_existing_users(users)
           @current_course.course_users.build(user: user[:user], name: user[:name],
                                              role: user[:role], phantom: user[:phantom],
                                              timeline_algorithm: @current_course.default_timeline_algorithm,
+                                             external_id: user[:external_id],
                                              creator: @current_user, updater: @current_user)
         @current_course.enrol_requests.pending.find_by(user: user[:user].id)&.destroy!
       end
@@ -113,7 +114,8 @@ def invite_new_users(users)
         new_invitations <<
           @current_course.invitations.build(name: user[:name], email: user[:email],
                                             role: user[:role], phantom: user[:phantom],
-                                            timeline_algorithm: user[:timeline_algorithm])
+                                            timeline_algorithm: user[:timeline_algorithm],
+                                            external_id: user[:external_id])
       end
     end
 
diff --git a/app/services/course/statistics/assessments_score_summary_download_service.rb b/app/services/course/statistics/assessments_score_summary_download_service.rb
index c42f4812517..bd31596a655 100644
--- a/app/services/course/statistics/assessments_score_summary_download_service.rb
+++ b/app/services/course/statistics/assessments_score_summary_download_service.rb
@@ -46,6 +46,7 @@ def load_total_grades
 
     @submission_grade_hash = submission_grade_hash
     @all_students = @course.course_users.students.order_alphabetically.preload(user: :emails)
+    @include_external_id = @all_students.any? { |s| s.external_id.present? }
   end
 
   def submission_grade_hash
@@ -67,15 +68,15 @@ def download_score_summary(csv)
       I18n.t('csv.score_summary.headers.name'),
       I18n.t('csv.score_summary.headers.email'),
       I18n.t('csv.score_summary.headers.type'),
-      *@assessments.map(&:title)
+      *@assessments.map(&:title),
+      *(@include_external_id ? [I18n.t('csv.score_summary.headers.external_id')] : [])
     ]
 
     # content
     @all_students.each do |student|
       csv << [student.name, student.user.email, student.phantom? ? 'phantom' : 'normal',
-              *@assessments.flat_map do |assessment|
-                @submission_grade_hash[[student.id, assessment.id]] || ''
-              end]
+              *@assessments.flat_map { |a| @submission_grade_hash[[student.id, a.id]] || '' },
+              *(@include_external_id ? [student.external_id || ''] : [])]
     end
   end
 end
diff --git a/app/services/course/user_registration_service.rb b/app/services/course/user_registration_service.rb
index c397c19f025..11f0ecb9dac 100644
--- a/app/services/course/user_registration_service.rb
+++ b/app/services/course/user_registration_service.rb
@@ -52,10 +52,12 @@ def find_or_create_course_user!(registration, invitation = nil)
     role = invitation.try(:role) || :student
     phantom = invitation.try(:phantom) || false
     timeline_algorithm = invitation.try(:timeline_algorithm) || registration.course.default_timeline_algorithm
+    external_id = invitation.try(:external_id) || nil
 
     registration.course_user =
       CourseUser.find_or_create_by!(course: registration.course, user: registration.user,
-                                    name: name, role: role, phantom: phantom, timeline_algorithm: timeline_algorithm)
+                                    name: name, role: role, phantom: phantom, timeline_algorithm: timeline_algorithm,
+                                    external_id: external_id)
   end
 
   # Claims a given registration code. The correct type of code is deduced from the code itself and
diff --git a/app/views/course/user_invitations/_course_user_invitation_list_data.json.jbuilder b/app/views/course/user_invitations/_course_user_invitation_list_data.json.jbuilder
index fa5593e906f..886056a10d8 100644
--- a/app/views/course/user_invitations/_course_user_invitation_list_data.json.jbuilder
+++ b/app/views/course/user_invitations/_course_user_invitation_list_data.json.jbuilder
@@ -3,6 +3,7 @@
 json.id invitation.id
 json.name invitation.name
 json.email invitation.email
+json.externalId invitation.external_id
 json.role invitation.role
 json.phantom invitation.phantom
 json.timelineAlgorithm invitation.timeline_algorithm
diff --git a/app/views/course/user_invitations/_invitation_result_data.json.jbuilder b/app/views/course/user_invitations/_invitation_result_data.json.jbuilder
index 683da5be9da..421b05dc99d 100644
--- a/app/views/course/user_invitations/_invitation_result_data.json.jbuilder
+++ b/app/views/course/user_invitations/_invitation_result_data.json.jbuilder
@@ -4,6 +4,7 @@ json.newInvitations new_invitations.each do |invitation|
   json.id invitation.id
   json.name invitation.name
   json.email invitation.email
+  json.externalId invitation.external_id
   json.role invitation.role
   json.phantom invitation.phantom
   json.sentAt invitation.sent_at
@@ -13,6 +14,7 @@ json.existingInvitations existing_invitations.each do |invitation|
   json.id invitation.id
   json.name invitation.name
   json.email invitation.email
+  json.externalId invitation.external_id
   json.role invitation.role
   json.phantom invitation.phantom
   json.sentAt invitation.sent_at
@@ -22,6 +24,7 @@ json.newCourseUsers new_course_users.each do |course_user|
   json.id course_user.id if course_user.id
   json.name course_user.name.strip
   json.email course_user.user.email
+  json.externalId course_user.external_id
   json.role course_user.role
   json.phantom course_user.phantom?
 end
@@ -30,6 +33,7 @@ json.existingCourseUsers existing_course_users.each do |course_user|
   json.id course_user.id if course_user.id
   json.name course_user.name.strip
   json.email course_user.user.email
+  json.externalId course_user.external_id
   json.role course_user.role
   json.phantom course_user.phantom?
 end
@@ -38,6 +42,8 @@ json.duplicateUsers duplicate_users.each do |duplicate_user, index|
   json.id index
   json.name duplicate_user[:name]
   json.email duplicate_user[:email]
+  json.externalId duplicate_user[:external_id]
   json.role duplicate_user[:role]
   json.phantom duplicate_user[:phantom]
+  json.reason duplicate_user[:reason]
 end
diff --git a/app/views/course/users/_user_list_data.json.jbuilder b/app/views/course/users/_user_list_data.json.jbuilder
index 558dcb055e7..bd5400d113a 100644
--- a/app/views/course/users/_user_list_data.json.jbuilder
+++ b/app/views/course/users/_user_list_data.json.jbuilder
@@ -16,3 +16,4 @@ json.timelineAlgorithm course_user.timeline_algorithm if should_show_timeline
 
 json.role course_user.role
 json.phantom course_user.phantom? if should_show_phantom
+json.externalId course_user.external_id
diff --git a/client/app/assets/templates/course-user-invitation-template-no-timeline.csv b/client/app/assets/templates/course-user-invitation-template-no-timeline.csv
new file mode 100644
index 00000000000..6874f099d4c
--- /dev/null
+++ b/client/app/assets/templates/course-user-invitation-template-no-timeline.csv
@@ -0,0 +1,3 @@
+Name,Email,Role,Phantom,ExternalId
+John,test1@example.com,student,y,a01234567
+Mary,test2@example.com,teaching_assistant,n,a01234568
diff --git a/client/app/assets/templates/course-user-invitation-template.csv b/client/app/assets/templates/course-user-invitation-template.csv
index 05923277b94..4e0ee4106ed 100644
--- a/client/app/assets/templates/course-user-invitation-template.csv
+++ b/client/app/assets/templates/course-user-invitation-template.csv
@@ -1,3 +1,3 @@
-Name,Email,Role,Phantom,Timeline
-John,test1@example.com,student,y,otot
-Mary,test2@example.com,teaching_assistant,n,fixed
\ No newline at end of file
+Name,Email,Role,Phantom,Timeline,ExternalId
+John,test1@example.com,student,y,otot,a01234567
+Mary,test2@example.com,teaching_assistant,n,fixed,a01234568
\ No newline at end of file
diff --git a/client/app/bundles/course/helper/index.ts b/client/app/bundles/course/helper/index.ts
index 84b9b02d036..01eb52ba487 100644
--- a/client/app/bundles/course/helper/index.ts
+++ b/client/app/bundles/course/helper/index.ts
@@ -1,5 +1,6 @@
 import courseDefaultLogoUrl from 'assets/images/course-default-logo.svg?url';
 import courseUserInvitationTemplateUrl from 'assets/templates/course-user-invitation-template.csv?url';
+import courseUserInvitationTemplateNoTimelineUrl from 'assets/templates/course-user-invitation-template-no-timeline.csv?url';
 
 export const getCourseLogoUrl = (url?: string | null): string => {
   if (!url) {
@@ -8,6 +9,10 @@ export const getCourseLogoUrl = (url?: string | null): string => {
   return url;
 };
 
-export const getCourseUserInviteTemplatePath = (): string => {
-  return courseUserInvitationTemplateUrl;
+export const getCourseUserInviteTemplatePath = (
+  hasPersonalTimelines: boolean,
+): string => {
+  return hasPersonalTimelines
+    ? courseUserInvitationTemplateUrl
+    : courseUserInvitationTemplateNoTimelineUrl;
 };
diff --git a/client/app/bundles/course/user-invitations/components/buttons/InvitationActionButtons.tsx b/client/app/bundles/course/user-invitations/components/buttons/InvitationActionButtons.tsx
index 303bb904153..cccd846fae1 100644
--- a/client/app/bundles/course/user-invitations/components/buttons/InvitationActionButtons.tsx
+++ b/client/app/bundles/course/user-invitations/components/buttons/InvitationActionButtons.tsx
@@ -27,7 +27,7 @@ const translations = defineMessages({
   },
   resendFailure: {
     id: 'course.userInvitations.InvitationActionButtons.resendFailure',
-    defaultMessage: 'Failed to resend invitation - {error}',
+    defaultMessage: 'Failed to resend invitation.',
   },
   deletionTooltip: {
     id: 'course.userInvitations.InvitationActionButtons.deletionTooltip',
@@ -46,6 +46,10 @@ const translations = defineMessages({
     id: 'course.userInvitations.InvitationActionButtons.deletionFailure',
     defaultMessage: 'Failed to delete user - {error}',
   },
+  deletionFailureGeneric: {
+    id: 'course.userInvitations.InvitationActionButtons.deletionFailureGeneric',
+    defaultMessage: 'Failed to delete user.',
+  },
 });
 
 const InvitationActionButtons: FC<Props> = (props) => {
@@ -65,15 +69,9 @@ const InvitationActionButtons: FC<Props> = (props) => {
           }),
         );
       })
-      .catch((error) => {
-        const errorMessage = error.response?.data?.errors
-          ? error.response.data.errors
-          : '';
-        toast.error(
-          t(translations.resendFailure, {
-            error: errorMessage,
-          }),
-        );
+      .catch(() => {
+        // resend failure endpoints return head :bad_request with no body
+        toast.error(t(translations.resendFailure));
       })
       .finally(() => setIsResending(false));
   };
@@ -90,14 +88,16 @@ const InvitationActionButtons: FC<Props> = (props) => {
       })
       .catch((error) => {
         setIsDeleting(false);
-        const errorMessage = error.response?.data?.errors
-          ? error.response.data.errors
-          : '';
-        toast.error(
-          t(translations.deletionFailure, {
-            error: errorMessage,
-          }),
-        );
+        const rawErrors = error.response?.data?.errors;
+        let errorList: string[];
+        if (Array.isArray(rawErrors)) errorList = rawErrors;
+        else if (typeof rawErrors === 'string') errorList = [rawErrors];
+        else errorList = [];
+        if (errorList[0]) {
+          toast.error(t(translations.deletionFailure, { error: errorList[0] }));
+        } else {
+          toast.error(t(translations.deletionFailureGeneric));
+        }
         throw error;
       });
   };
diff --git a/client/app/bundles/course/user-invitations/components/forms/IndividualInvitation.tsx b/client/app/bundles/course/user-invitations/components/forms/IndividualInvitation.tsx
index 7e7e88c168d..25b7bfb2cdc 100644
--- a/client/app/bundles/course/user-invitations/components/forms/IndividualInvitation.tsx
+++ b/client/app/bundles/course/user-invitations/components/forms/IndividualInvitation.tsx
@@ -134,6 +134,20 @@ const IndividualInvitation: FC<Props> = (props) => {
           )}
         />
       )}
+      <Controller
+        control={fieldsConfig.control}
+        name={`invitations.${index}.externalId`}
+        render={({ field, fieldState }): JSX.Element => (
+          <FormTextField
+            field={field}
+            fieldState={fieldState}
+            id={`externalId-${index}`}
+            label={`${t(tableTranslations.externalId)} (${t(tableTranslations.optional)})`}
+            sx={styles.textInput}
+            variant="standard"
+          />
+        )}
+      />
       <Controller
         control={fieldsConfig.control}
         name={`invitations.${index}.phantom`}
diff --git a/client/app/bundles/course/user-invitations/components/forms/IndividualInviteForm.tsx b/client/app/bundles/course/user-invitations/components/forms/IndividualInviteForm.tsx
index 9564b8aff0b..9980c04d932 100644
--- a/client/app/bundles/course/user-invitations/components/forms/IndividualInviteForm.tsx
+++ b/client/app/bundles/course/user-invitations/components/forms/IndividualInviteForm.tsx
@@ -1,6 +1,6 @@
 import { FC, useEffect, useState } from 'react';
 import { useFieldArray, useForm } from 'react-hook-form';
-import { injectIntl, WrappedComponentProps } from 'react-intl';
+import { defineMessages } from 'react-intl';
 import { yupResolver } from '@hookform/resolvers/yup';
 import {
   IndividualInvites,
@@ -12,8 +12,8 @@ import * as yup from 'yup';
 import ErrorText from 'lib/components/core/ErrorText';
 import { useAppDispatch, useAppSelector } from 'lib/hooks/store';
 import toast from 'lib/hooks/toast';
+import useTranslation from 'lib/hooks/useTranslation';
 import formTranslations from 'lib/translations/form';
-import messagesTranslations from 'lib/translations/messages';
 
 import { inviteUsersFromForm } from '../../operations';
 import {
@@ -23,7 +23,18 @@ import {
 
 import IndividualInvitations from './IndividualInvitations';
 
-interface Props extends WrappedComponentProps {
+const translations = defineMessages({
+  failure: {
+    id: 'course.userInvitations.IndividualInviteForm.failure',
+    defaultMessage: 'Failed to invite users. {error}',
+  },
+  failureGeneric: {
+    id: 'course.userInvitations.IndividualInviteForm.failureGeneric',
+    defaultMessage: 'Failed to invite users. You may reload and try again.',
+  },
+});
+
+interface Props {
   openResultDialog: (invitationResult: InvitationResult) => void;
 }
 
@@ -46,7 +57,8 @@ const validationSchema = yup.object({
 });
 
 const IndividualInviteForm: FC<Props> = (props) => {
-  const { openResultDialog, intl } = props;
+  const { openResultDialog } = props;
+  const { t } = useTranslation();
   const [isLoading, setIsLoading] = useState(false);
   const dispatch = useAppDispatch();
   const sharedData = useAppSelector(getManageCourseUsersSharedData);
@@ -109,8 +121,22 @@ const IndividualInviteForm: FC<Props> = (props) => {
         reset(initialValues);
         openResultDialog(response);
       })
-      .catch(() => {
-        toast.error(intl.formatMessage(messagesTranslations.formUpdateError));
+      .catch((error) => {
+        const rawErrors = error.response?.data?.errors;
+        let errorList: string[];
+        if (Array.isArray(rawErrors)) errorList = rawErrors;
+        else if (typeof rawErrors === 'string') errorList = [rawErrors];
+        else errorList = [];
+        const first = errorList[0];
+        const overflow =
+          errorList.length > 1 ? ` (and ${errorList.length - 1} more)` : '';
+        if (first) {
+          toast.error(t(translations.failure, { error: first + overflow }), {
+            autoClose: false,
+          });
+        } else {
+          toast.error(t(translations.failureGeneric));
+        }
       })
       .finally(() => {
         setIsLoading(false);
@@ -139,4 +165,4 @@ const IndividualInviteForm: FC<Props> = (props) => {
   );
 };
 
-export default injectIntl(IndividualInviteForm);
+export default IndividualInviteForm;
diff --git a/client/app/bundles/course/user-invitations/components/misc/InvitationResultDialog.tsx b/client/app/bundles/course/user-invitations/components/misc/InvitationResultDialog.tsx
index e1958786c5f..8c6260c4434 100644
--- a/client/app/bundles/course/user-invitations/components/misc/InvitationResultDialog.tsx
+++ b/client/app/bundles/course/user-invitations/components/misc/InvitationResultDialog.tsx
@@ -52,11 +52,11 @@ const translations = defineMessages({
   duplicateInfo: {
     id: 'course.userInvitations.InvitationResultDialog.duplicateInfo',
     defaultMessage:
-      'Duplicate users were found in the invitation. Only the first instance of this user will be invited.',
+      'Duplicate users were found in the invitation. Only the first instance of each user will be invited.',
   },
   duplicateUsers: {
     id: 'course.userInvitations.InvitationResultDialog.duplicateUsers',
-    defaultMessage: 'Users with Duplicate Emails ({count})',
+    defaultMessage: 'Duplicate Users ({count})',
   },
   existingCourseUsersInfo: {
     id: 'course.userInvitations.InvitationResultDialog.existingCourseUsersInfo',
diff --git a/client/app/bundles/course/user-invitations/components/tables/InvitationResultInvitationsTable.tsx b/client/app/bundles/course/user-invitations/components/tables/InvitationResultInvitationsTable.tsx
index 2ee9ce1cd37..fb70f544196 100644
--- a/client/app/bundles/course/user-invitations/components/tables/InvitationResultInvitationsTable.tsx
+++ b/client/app/bundles/course/user-invitations/components/tables/InvitationResultInvitationsTable.tsx
@@ -21,6 +21,8 @@ const InvitationResultInvitationsTable: FC<Props> = (props) => {
 
   if (invitations && invitations.length === 0) return null;
 
+  const showExternalId = invitations.some((i) => i.externalId != null);
+
   const options: TableOptions = {
     download: true,
     filter: false,
@@ -69,6 +71,18 @@ const InvitationResultInvitationsTable: FC<Props> = (props) => {
         sort: false,
       },
     },
+    ...(showExternalId
+      ? [
+          {
+            name: 'externalId',
+            label: t(tableTranslations.externalId),
+            options: {
+              alignCenter: true,
+              sort: false,
+            },
+          },
+        ]
+      : []),
     {
       name: 'phantom',
       label: t(tableTranslations.phantom),
diff --git a/client/app/bundles/course/user-invitations/components/tables/InvitationResultUsersTable.tsx b/client/app/bundles/course/user-invitations/components/tables/InvitationResultUsersTable.tsx
index 5164af7ec63..197dbe17f44 100644
--- a/client/app/bundles/course/user-invitations/components/tables/InvitationResultUsersTable.tsx
+++ b/client/app/bundles/course/user-invitations/components/tables/InvitationResultUsersTable.tsx
@@ -1,17 +1,30 @@
 import { FC, memo } from 'react';
+import { defineMessages } from 'react-intl';
 import { Typography } from '@mui/material';
 import equal from 'fast-deep-equal';
 import { TableColumns, TableOptions } from 'types/components/DataTable';
-import { CourseUserData } from 'types/course/courseUsers';
+import { CourseUserListData } from 'types/course/courseUsers';
+import { DuplicateReason } from 'types/course/userInvitations';
 
 import DataTable from 'lib/components/core/layouts/DataTable';
 import useTranslation from 'lib/hooks/useTranslation';
 import roleTranslations from 'lib/translations/course/users/roles';
 import tableTranslations from 'lib/translations/table';
 
+const translations = defineMessages({
+  duplicateEmail: {
+    id: 'course.userInvitations.InvitationResultUsersTable.duplicateEmail',
+    defaultMessage: 'Duplicate email',
+  },
+  duplicateExternalId: {
+    id: 'course.userInvitations.InvitationResultUsersTable.duplicateExternalId',
+    defaultMessage: 'Duplicate external ID',
+  },
+});
+
 interface Props {
   title: JSX.Element;
-  users: CourseUserData[];
+  users: Array<CourseUserListData & { reason?: DuplicateReason }>;
 }
 
 const InvitationResultUsersTable: FC<Props> = (props) => {
@@ -20,6 +33,9 @@ const InvitationResultUsersTable: FC<Props> = (props) => {
 
   if (users && users.length === 0) return null;
 
+  const showExternalId = users.some((u) => u.externalId != null);
+  const showReason = users.some((u) => u.reason != null);
+
   const options: TableOptions = {
     download: true,
     filter: false,
@@ -66,6 +82,44 @@ const InvitationResultUsersTable: FC<Props> = (props) => {
         sort: false,
       },
     },
+    ...(showExternalId
+      ? [
+          {
+            name: 'externalId',
+            label: t(tableTranslations.externalId),
+            options: {
+              alignCenter: true,
+              sort: false,
+            },
+          },
+        ]
+      : []),
+    ...(showReason
+      ? [
+          {
+            name: 'reason',
+            label: t(tableTranslations.reason),
+            options: {
+              alignCenter: false,
+              sort: false,
+              customBodyRenderLite: (dataIndex: number): JSX.Element => {
+                const user = users[dataIndex];
+                return (
+                  <Typography
+                    key={`reason-${user.id}`}
+                    className="invitation_result_user_reason"
+                    variant="body2"
+                  >
+                    {user.reason === 'duplicate_external_id'
+                      ? t(translations.duplicateExternalId)
+                      : t(translations.duplicateEmail)}
+                  </Typography>
+                );
+              },
+            },
+          },
+        ]
+      : []),
     {
       name: 'phantom',
       label: t(tableTranslations.phantom),
diff --git a/client/app/bundles/course/user-invitations/components/tables/UserInvitationsTable.tsx b/client/app/bundles/course/user-invitations/components/tables/UserInvitationsTable.tsx
index a033d128c9a..74d6604df88 100644
--- a/client/app/bundles/course/user-invitations/components/tables/UserInvitationsTable.tsx
+++ b/client/app/bundles/course/user-invitations/components/tables/UserInvitationsTable.tsx
@@ -135,6 +135,10 @@ const FailedChip: FC<{ sentAt: string }> = ({ sentAt }) => {
 const UserInvitationsTable: FC<Props> = (props) => {
   const { invitations } = props;
 
+  const showExternalId = invitations.some(
+    (invitation) => invitation.externalId != null,
+  );
+
   const { t } = useTranslation();
   const permissions = useAppSelector(getManageCourseUserPermissions);
 
@@ -158,6 +162,17 @@ const UserInvitationsTable: FC<Props> = (props) => {
       searchable: true,
       cell: (datum) => datum.email,
     },
+    ...(showExternalId
+      ? [
+          {
+            of: 'externalId',
+            title: t(tableTranslations.externalId ?? null),
+            sortable: false,
+            searchable: false,
+            cell: (datum) => datum.externalId ?? null,
+          } satisfies ColumnTemplate<InvitationRowData>,
+        ]
+      : []),
     {
       of: 'role',
       title: t(tableTranslations.role),
diff --git a/client/app/bundles/course/user-invitations/operations.ts b/client/app/bundles/course/user-invitations/operations.ts
index 49ad29fb24a..d9a0c409114 100644
--- a/client/app/bundles/course/user-invitations/operations.ts
+++ b/client/app/bundles/course/user-invitations/operations.ts
@@ -21,24 +21,32 @@ const formatInvitations = (invitations: InvitationPostData[]): FormData => {
   const payload = new FormData();
 
   invitations.forEach((invite, index) => {
-    ['name', 'email', 'role', 'phantom', 'timelineAlgorithm'].forEach(
-      (field) => {
-        if (invite[field] !== undefined && invite[field] !== null) {
-          let fieldName = field;
-          let value = invite[field];
-          if (field === 'timelineAlgorithm') {
-            fieldName = 'timeline_algorithm';
-          }
-          if (field === 'phantom') {
-            value = value ? 1 : 0;
-          }
-          payload.append(
-            `course[invitations_attributes][${index}][${fieldName}]`,
-            value,
-          );
+    [
+      'name',
+      'email',
+      'role',
+      'phantom',
+      'timelineAlgorithm',
+      'externalId',
+    ].forEach((field) => {
+      if (invite[field] !== undefined && invite[field] !== null) {
+        let fieldName = field;
+        let value = invite[field];
+        if (field === 'timelineAlgorithm') {
+          fieldName = 'timeline_algorithm';
         }
-      },
-    );
+        if (field === 'externalId') {
+          fieldName = 'external_id';
+        }
+        if (field === 'phantom') {
+          value = value ? 1 : 0;
+        }
+        payload.append(
+          `course[invitations_attributes][${index}][${fieldName}]`,
+          value,
+        );
+      }
+    });
   });
   return payload;
 };
diff --git a/client/app/bundles/course/user-invitations/pages/InviteUsersFileUpload/index.tsx b/client/app/bundles/course/user-invitations/pages/InviteUsersFileUpload/index.tsx
index 656ae9099a8..691ef3b828c 100644
--- a/client/app/bundles/course/user-invitations/pages/InviteUsersFileUpload/index.tsx
+++ b/client/app/bundles/course/user-invitations/pages/InviteUsersFileUpload/index.tsx
@@ -52,6 +52,16 @@ const translations = defineMessages({
       'Personal Timelines can be <code>[fixed, otot, stragglers, fomo]</code>,\
       with course default: {defaultTimelineAlgorithm} if omitted.',
   },
+  fileUploadInfoEmail: {
+    id: 'course.userInvitations.InviteUsersFileUpload.fileUploadInfoEmail',
+    defaultMessage:
+      'Each invitation must use a unique email address within the course. Duplicate emails will be skipped.',
+  },
+  fileUploadInfoExternalId: {
+    id: 'course.userInvitations.InviteUsersFileUpload.fileUploadInfoExternalId',
+    defaultMessage:
+      'External ID is an optional field for linking a user to an external system. If provided, it must be unique within the course — duplicate external IDs will be skipped.',
+  },
   exampleHeader: {
     id: 'course.userInvitations.InviteUsersFileUpload.exampleHeader',
     defaultMessage: 'Example ',
@@ -63,22 +73,27 @@ const translations = defineMessages({
   fileUploadExample: {
     id: 'course.userInvitations.InviteUsersFileUpload.fileUploadExample',
     defaultMessage:
-      'Name,Email[,Role,Phantom]' +
-      '{br}John,test1@example.org[,student,y]' +
-      '{br}Mary,test2@example.org[,teaching_assistant,n]',
+      'Name,Email,Role,Phantom,ExternalId' +
+      '{br}John,test1@example.org[,student,y,A0123456' +
+      '{br}Mary,test2@example.org[,teaching_assistant,n,A0123457',
   },
   fileUploadExamplePersonalTimeline: {
     id: 'course.userInvitations.InviteUsersFileUpload.fileUploadExamplePersonalTimeline',
     defaultMessage:
-      'Name,Email[,Role,Phantom,PersonalTimeline]' +
-      '{br}John,test1@example.org[,student,y,otot]' +
-      '{br}Mary,test2@example.org[,teaching_assistant,n,fixed]',
+      'Name,Email,Role,Phantom,PersonalTimeline,ExternalId' +
+      '{br}John,test1@example.org,student,y,otot,A0123456' +
+      '{br}Mary,test2@example.org,teaching_assistant,n,fixed,A0123457',
   },
   failure: {
     id: 'course.userInvitations.InviteUsersFileUpload.failure',
     defaultMessage:
       'Failed to invite users. Please ensure your data is formatted correctly - {error}',
   },
+  failureGeneric: {
+    id: 'course.userInvitations.InviteUsersFileUpload.failureGeneric',
+    defaultMessage:
+      'Failed to invite users. Please ensure your data is formatted correctly.',
+  },
 });
 
 const InviteUsersFileUpload: FC<Props> = (props) => {
@@ -102,15 +117,21 @@ const InviteUsersFileUpload: FC<Props> = (props) => {
         openResultDialog(response);
       })
       .catch((error) => {
-        const errorMessage = error.response?.data?.errors
-          ? error.response.data.errors
-          : '';
-        toast.error(
-          t(translations.failure, {
-            error: errorMessage,
-          }),
-          { autoClose: false },
-        );
+        const rawErrors = error.response?.data?.errors;
+        let errorList: string[];
+        if (Array.isArray(rawErrors)) errorList = rawErrors;
+        else if (typeof rawErrors === 'string') errorList = [rawErrors];
+        else errorList = [];
+        const first = errorList[0];
+        const overflow =
+          errorList.length > 1 ? ` (and ${errorList.length - 1} more)` : '';
+        if (first) {
+          toast.error(t(translations.failure, { error: first + overflow }), {
+            autoClose: false,
+          });
+        } else {
+          toast.error(t(translations.failureGeneric), { autoClose: false });
+        }
       });
   };
 
@@ -118,6 +139,11 @@ const InviteUsersFileUpload: FC<Props> = (props) => {
     <>
       <Typography variant="body2">{t(translations.fileUploadInfo)}</Typography>
       <ul>
+        <li>
+          <Typography variant="body2">
+            {t(translations.fileUploadInfoEmail)}
+          </Typography>
+        </li>
         <li>
           <Typography variant="body2">
             <FormattedMessage {...translations.fileUploadInfoRole} />
@@ -141,12 +167,19 @@ const InviteUsersFileUpload: FC<Props> = (props) => {
             </Typography>
           </li>
         )}
+        <li>
+          <Typography variant="body2">
+            <FormattedMessage {...translations.fileUploadInfoExternalId} />
+          </Typography>
+        </li>
       </ul>
       <Typography variant="body2">
         <strong>{t(translations.exampleHeader)}</strong>
         <Link
           download="template.csv"
-          href={getCourseUserInviteTemplatePath()}
+          href={getCourseUserInviteTemplatePath(
+            permissions.canManagePersonalTimes,
+          )}
           opensInNewTab
           style={{ textDecoration: 'none', cursor: 'pointer' }}
         >
diff --git a/client/app/bundles/course/users/components/tables/ManageUsersTable/ExternalIdField.tsx b/client/app/bundles/course/users/components/tables/ManageUsersTable/ExternalIdField.tsx
new file mode 100644
index 00000000000..b049d26efbe
--- /dev/null
+++ b/client/app/bundles/course/users/components/tables/ManageUsersTable/ExternalIdField.tsx
@@ -0,0 +1,72 @@
+import { memo } from 'react';
+import equal from 'fast-deep-equal';
+import { CourseUserMiniEntity } from 'types/course/courseUsers';
+
+import { updateUser } from 'bundles/course/users/operations';
+import InlineEditTextField from 'lib/components/form/fields/DataTableInlineEditable/TextField';
+import { useAppDispatch } from 'lib/hooks/store';
+import toast from 'lib/hooks/toast';
+import useTranslation from 'lib/hooks/useTranslation';
+
+import translations from '../../../translations';
+
+interface ExternalIdFieldProps {
+  for: CourseUserMiniEntity;
+}
+
+const ExternalIdField = (props: ExternalIdFieldProps): JSX.Element => {
+  const { for: user } = props;
+
+  const dispatch = useAppDispatch();
+
+  const { t } = useTranslation();
+
+  const handleIdUpdate = (id: string): Promise<void> => {
+    return dispatch(updateUser(user.id, { externalId: id || null }))
+      .then(() => {
+        if (!id && user.externalId) {
+          toast.success(t(translations.deleteIdSuccess));
+        } else if (id && !user.externalId) {
+          toast.success(t(translations.addIdSuccess, { newId: id }));
+        } else {
+          toast.success(
+            t(translations.changeIdSuccess, {
+              oldId: user.externalId ?? '',
+              newId: id,
+            }),
+          );
+        }
+      })
+      .catch((error) => {
+        if (!id && user.externalId) {
+          toast.error(t(translations.deleteIdFailure));
+        } else if (id && !user.externalId) {
+          toast.error(t(translations.addIdFailure, { newId: id }));
+        } else {
+          toast.error(
+            t(translations.changeIdFailure, {
+              oldId: user.externalId ?? '',
+              newId: id,
+            }),
+          );
+        }
+        throw error;
+      });
+  };
+
+  return (
+    <InlineEditTextField
+      key={user.id}
+      allowEmpty
+      className="course_user_external_id"
+      onUpdate={(newId): Promise<void> => handleIdUpdate(newId)}
+      updateValue={(): void => {}}
+      value={user.externalId ?? ''}
+      variant="standard"
+    />
+  );
+};
+
+export default memo(ExternalIdField, (prevProps, nextProps) =>
+  equal(prevProps.for.externalId, nextProps.for.externalId),
+);
diff --git a/client/app/bundles/course/users/components/tables/ManageUsersTable/UserNameField.tsx b/client/app/bundles/course/users/components/tables/ManageUsersTable/UserNameField.tsx
index 1eee7291d5d..9787df3e0b8 100644
--- a/client/app/bundles/course/users/components/tables/ManageUsersTable/UserNameField.tsx
+++ b/client/app/bundles/course/users/components/tables/ManageUsersTable/UserNameField.tsx
@@ -46,6 +46,7 @@ const UserNameField = (props: UserNameFieldProps): JSX.Element => {
   return (
     <InlineEditTextField
       key={user.id}
+      allowEmpty={false}
       className={`course_user_name ${user.isSuspended ? 'text-neutral-400' : ''}`}
       onUpdate={(newName): Promise<void> => handleNameUpdate(newName)}
       updateValue={(): void => {}}
diff --git a/client/app/bundles/course/users/components/tables/ManageUsersTable/index.tsx b/client/app/bundles/course/users/components/tables/ManageUsersTable/index.tsx
index 2343c8f10f2..31da6ed8c16 100644
--- a/client/app/bundles/course/users/components/tables/ManageUsersTable/index.tsx
+++ b/client/app/bundles/course/users/components/tables/ManageUsersTable/index.tsx
@@ -15,6 +15,7 @@ import translations from '../../../translations';
 
 import ActiveTableToolbar from './ActiveTableToolbar';
 import AlgorithmMenu from './AlgorithmMenu';
+import ExternalIdField from './ExternalIdField';
 import PhantomSwitch from './PhantomSwitch';
 import RoleMenu from './RoleMenu';
 import TimelineMenu from './TimelineMenu';
@@ -58,6 +59,14 @@ const ManageUsersTable = (props: ManageUsersTableProps): JSX.Element => {
       cell: (user) => <UserNameField for={user} />,
       csvDownloadable: true,
     },
+    {
+      of: 'externalId',
+      title: t(tableTranslations.externalId),
+      sortable: false,
+      searchable: true,
+      cell: (user) => <ExternalIdField for={user} />,
+      csvDownloadable: true,
+    },
     {
       of: 'email',
       sortable: true,
diff --git a/client/app/bundles/course/users/operations.ts b/client/app/bundles/course/users/operations.ts
index 81b7c216d50..3d9990666da 100644
--- a/client/app/bundles/course/users/operations.ts
+++ b/client/app/bundles/course/users/operations.ts
@@ -40,6 +40,7 @@ const formatUpdateUser = (
       role: data.role,
       reference_timeline_id: data.referenceTimelineId,
       timeline_algorithm: data.timelineAlgorithm,
+      external_id: data.externalId,
     },
   };
 };
diff --git a/client/app/bundles/course/users/translations.ts b/client/app/bundles/course/users/translations.ts
index 0272f787c96..cb7aec0635d 100644
--- a/client/app/bundles/course/users/translations.ts
+++ b/client/app/bundles/course/users/translations.ts
@@ -17,6 +17,30 @@ const translations = defineMessages({
     id: 'course.users.ManageUsersTable.renameFailure',
     defaultMessage: 'Failed to rename {oldName} to {newName}',
   },
+  changeIdSuccess: {
+    id: 'course.users.ManageUsersTable.changeIdSuccess',
+    defaultMessage: 'ID was changed from {oldId} to {newId}',
+  },
+  addIdSuccess: {
+    id: 'course.users.ManageUsersTable.addIdSuccess',
+    defaultMessage: 'External ID set to {newId}',
+  },
+  deleteIdSuccess: {
+    id: 'course.users.ManageUsersTable.deleteIdSuccess',
+    defaultMessage: 'External ID deleted',
+  },
+  addIdFailure: {
+    id: 'course.users.ManageUsersTable.addIdFailure',
+    defaultMessage: 'Failed to set External ID to {newId}',
+  },
+  deleteIdFailure: {
+    id: 'course.users.ManageUsersTable.deleteIdFailure',
+    defaultMessage: 'Failed to delete External ID',
+  },
+  changeIdFailure: {
+    id: 'course.users.ManageUsersTable.changeIdFailure',
+    defaultMessage: 'Failed to change ID from {oldId} to {newId}',
+  },
   phantomSuccess: {
     id: 'course.users.ManageUsersTable.phantomSuccess',
     defaultMessage:
diff --git a/client/app/lib/components/form/fields/DataTableInlineEditable/TextField.tsx b/client/app/lib/components/form/fields/DataTableInlineEditable/TextField.tsx
index 7d416acfb7e..0820e28e755 100644
--- a/client/app/lib/components/form/fields/DataTableInlineEditable/TextField.tsx
+++ b/client/app/lib/components/form/fields/DataTableInlineEditable/TextField.tsx
@@ -19,6 +19,7 @@ interface Props {
   link?: string;
   onUpdate?: (newValue: string) => Promise<void>;
   alwaysEditable?: boolean;
+  allowEmpty?: boolean;
 }
 
 const styles = {
@@ -52,6 +53,7 @@ const InlineEditTextField: FC<Props> = (props): JSX.Element | null => {
     link,
     onUpdate,
     alwaysEditable = false,
+    allowEmpty = false,
     ...custom
   } = props;
   const [controlledVal, setControlledVal] = useState(value);
@@ -68,7 +70,7 @@ const InlineEditTextField: FC<Props> = (props): JSX.Element | null => {
 
   const handleSave = (): void => {
     setIsSaving(true);
-    if (controlledVal.trim() === '') {
+    if (!allowEmpty && controlledVal.trim() === '') {
       setHelperText('Cannot be empty.');
       setIsSaving(false);
       return;
diff --git a/client/app/lib/translations/table.ts b/client/app/lib/translations/table.ts
index c15b5316d8a..7136e9c9d73 100644
--- a/client/app/lib/translations/table.ts
+++ b/client/app/lib/translations/table.ts
@@ -37,6 +37,10 @@ const translations = defineMessages({
     id: 'lib.translations.table.column.timelineAlgorithm',
     defaultMessage: 'Algorithm',
   },
+  externalId: {
+    id: 'lib.translations.table.column.externalId',
+    defaultMessage: 'External ID',
+  },
   invitationSentAt: {
     id: 'lib.translations.table.column.invitationSentAt',
     defaultMessage: 'Invitation Sent At',
@@ -213,6 +217,10 @@ const translations = defineMessages({
     id: 'lib.translations.table.column.groups',
     defaultMessage: 'Group(s)',
   },
+  optional: {
+    id: 'lib.translations.table.column.optional',
+    defaultMessage: 'Optional',
+  },
 });
 
 export default translations;
diff --git a/client/app/types/course/courseUsers.ts b/client/app/types/course/courseUsers.ts
index 2b7adfeac76..9affb2d7bf7 100644
--- a/client/app/types/course/courseUsers.ts
+++ b/client/app/types/course/courseUsers.ts
@@ -52,6 +52,7 @@ export interface CourseUserListData extends CourseUserBasicListData {
   phantom?: boolean;
   isSuspended?: boolean;
   timelineAlgorithm?: TimelineAlgorithm;
+  externalId?: string | null;
 }
 
 export interface CourseUserBasicMiniEntity {
@@ -69,6 +70,7 @@ export interface CourseUserMiniEntity extends CourseUserBasicMiniEntity {
   email: CourseUserListData['email'];
   role: CourseUserListData['role'];
   timelineAlgorithm?: CourseUserListData['timelineAlgorithm'];
+  externalId?: CourseUserListData['externalId'];
   referenceTimelineId?: number | null;
   groups?: string[];
 }
@@ -118,6 +120,7 @@ export interface UpdateCourseUserPatchData {
     timeline_algorithm?: TimelineAlgorithm;
     reference_timeline_id?: number | null;
     role?: CourseUserRole;
+    external_id?: string | null;
   };
 }
 
diff --git a/client/app/types/course/userInvitations.ts b/client/app/types/course/userInvitations.ts
index c75fee13e8e..65cdd2284aa 100644
--- a/client/app/types/course/userInvitations.ts
+++ b/client/app/types/course/userInvitations.ts
@@ -1,4 +1,8 @@
-import { CourseUserData, CourseUserRole } from './courseUsers';
+import {
+  CourseUserData,
+  CourseUserListData,
+  CourseUserRole,
+} from './courseUsers';
 import { TimelineAlgorithm } from './personalTimes';
 
 export interface InvitationFileEntity {
@@ -7,8 +11,14 @@ export interface InvitationFileEntity {
   file?: Blob;
 }
 
+export type DuplicateReason = 'duplicate_email' | 'duplicate_external_id';
+
+export interface DuplicateUserData extends CourseUserListData {
+  reason?: DuplicateReason;
+}
+
 export interface InvitationResult {
-  duplicateUsers?: CourseUserData[];
+  duplicateUsers?: DuplicateUserData[];
   existingCourseUsers?: CourseUserData[];
   existingInvitations?: InvitationListData[];
   newCourseUsers?: CourseUserData[];
@@ -22,6 +32,7 @@ export interface IndividualInvite {
   id?: string;
   name: string;
   email: string;
+  externalId?: string;
   role: string;
   phantom: boolean;
   timelineAlgorithm?: string;
@@ -34,6 +45,7 @@ export interface InvitationPostData {
   id?: string | undefined;
   name: string;
   email: string;
+  externalId?: string | null;
   role: string;
   phantom: boolean;
   timelineAlgorithm?: string | undefined;
@@ -44,6 +56,7 @@ export interface InvitationsPostData {
     id?: string | undefined;
     name: string;
     email: string;
+    externalId?: string | null;
     role: string;
     phantom: boolean;
     timelineAlgorithm?: string | undefined;
@@ -54,6 +67,7 @@ export interface InvitationMiniEntity {
   id: number;
   name: string;
   email: string;
+  externalId: string | null;
   role: CourseUserRole;
   phantom: boolean;
   timelineAlgorithm?: TimelineAlgorithm;
@@ -68,6 +82,7 @@ export interface InvitationListData {
   id: number;
   name: string;
   email: string;
+  externalId: string | null;
   role: CourseUserRole;
   phantom: boolean;
   timelineAlgorithm?: TimelineAlgorithm;
diff --git a/client/locales/en.json b/client/locales/en.json
index 9010ae111cf..9f18b48ecae 100644
--- a/client/locales/en.json
+++ b/client/locales/en.json
@@ -6889,10 +6889,10 @@
     "defaultMessage": "Close"
   },
   "course.userInvitations.InvitationResultDialog.duplicateInfo": {
-    "defaultMessage": "Duplicate users were found in the invitation. Only the first instance of this user will be invited."
+    "defaultMessage": "Duplicate users were found in the invitation. Only the first instance of each user will be invited."
   },
   "course.userInvitations.InvitationResultDialog.duplicateUsers": {
-    "defaultMessage": "Users with Duplicate Emails ({count})"
+    "defaultMessage": "Duplicate Users ({count})"
   },
   "course.userInvitations.InvitationResultDialog.existingCourseUsers": {
     "defaultMessage": "Existing Course Users ({count})"
@@ -6915,6 +6915,12 @@
   "course.userInvitations.InvitationResultDialog.newInvitations": {
     "defaultMessage": "New Invitations ({count})"
   },
+  "course.userInvitations.InvitationResultUsersTable.duplicateEmail": {
+    "defaultMessage": "Duplicate email"
+  },
+  "course.userInvitations.InvitationResultUsersTable.duplicateExternalId": {
+    "defaultMessage": "Duplicate external ID"
+  },
   "course.userInvitations.InvitationsBarChart.accepted": {
     "defaultMessage": "Accepted Invitations"
   },
@@ -6945,11 +6951,20 @@
   "course.userInvitations.InviteUsersFileUpload.failure": {
     "defaultMessage": "Failed to invite users. Please ensure your data is formatted correctly - {error}"
   },
+  "course.userInvitations.InviteUsersFileUpload.failureGeneric": {
+    "defaultMessage": "Failed to invite users. Please ensure your data is formatted correctly."
+  },
   "course.userInvitations.InviteUsersFileUpload.fileUploadExample": {
-    "defaultMessage": "Name,Email[,Role,Phantom]{br}John,test1@example.org[,student,y]{br}Mary,test2@example.org[,teaching_assistant,n]"
+    "defaultMessage": "Name,Email,Role,Phantom,ExternalId{br}John,test1@example.org,student,y,A0123456{br}Mary,test2@example.org,teaching_assistant,n,A0123457"
   },
   "course.userInvitations.InviteUsersFileUpload.fileUploadExamplePersonalTimeline": {
-    "defaultMessage": "Name,Email[,Role,Phantom,PersonalTimeline]{br}John,test1@example.org[,student,y,otot]{br}Mary,test2@example.org[,teaching_assistant,n,fixed]"
+    "defaultMessage": "Name,Email,Role,Phantom,PersonalTimeline,ExternalId{br}John,test1@example.org,student,y,otot,A0123456{br}Mary,test2@example.org,teaching_assistant,n,fixed,A0123457"
+  },
+  "course.userInvitations.InviteUsersFileUpload.fileUploadInfoEmail": {
+    "defaultMessage": "Each invitation must use a unique email address within the course. Duplicate emails will be skipped."
+  },
+  "course.userInvitations.InviteUsersFileUpload.fileUploadInfoExternalId": {
+    "defaultMessage": "External ID is an optional field for linking a user to an external system. If provided, it must be unique within the course — duplicate external IDs will be skipped."
   },
   "course.userInvitations.InviteUsersFileUpload.fileUploadInfo": {
     "defaultMessage": "Upload a .csv file with the following format:"
@@ -6981,6 +6996,9 @@
   "course.userInvitations.InvitationActionButtons.deletionFailure": {
     "defaultMessage": "Failed to delete user - {error}"
   },
+  "course.userInvitations.InvitationActionButtons.deletionFailureGeneric": {
+    "defaultMessage": "Failed to delete user."
+  },
   "course.userInvitations.InvitationActionButtons.deletionSuccess": {
     "defaultMessage": "Invitation for {name} was deleted."
   },
@@ -6988,7 +7006,7 @@
     "defaultMessage": "Delete Invitation"
   },
   "course.userInvitations.InvitationActionButtons.resendFailure": {
-    "defaultMessage": "Failed to resend invitation - {error}"
+    "defaultMessage": "Failed to resend invitation."
   },
   "course.userInvitations.InvitationActionButtons.resendSuccess": {
     "defaultMessage": "Resent email invitation to {email}!"
@@ -7116,6 +7134,24 @@
   "course.users.ManageUsersTable.phantomSuccess": {
     "defaultMessage": "{name} {isPhantom, select, true {is now a phantom user} other {is now a normal user} }."
   },
+  "course.users.ManageUsersTable.addIdFailure": {
+    "defaultMessage": "Failed to set External ID to {newId}"
+  },
+  "course.users.ManageUsersTable.addIdSuccess": {
+    "defaultMessage": "External ID set to {newId}"
+  },
+  "course.users.ManageUsersTable.changeIdFailure": {
+    "defaultMessage": "Failed to change ID from {oldId} to {newId}"
+  },
+  "course.users.ManageUsersTable.changeIdSuccess": {
+    "defaultMessage": "ID was changed from {oldId} to {newId}"
+  },
+  "course.users.ManageUsersTable.deleteIdFailure": {
+    "defaultMessage": "Failed to delete External ID"
+  },
+  "course.users.ManageUsersTable.deleteIdSuccess": {
+    "defaultMessage": "External ID deleted"
+  },
   "course.users.ManageUsersTable.renameFailure": {
     "defaultMessage": "Failed to rename {oldName} to {newName}"
   },
@@ -8055,6 +8091,9 @@
   "lib.translations.table.column.email": {
     "defaultMessage": "Email"
   },
+  "lib.translations.table.column.externalId": {
+    "defaultMessage": "External ID"
+  },
   "lib.translations.table.column.endAt": {
     "defaultMessage": "End At"
   },
@@ -8106,6 +8145,9 @@
   "lib.translations.table.column.name": {
     "defaultMessage": "Name"
   },
+  "lib.translations.table.column.optional": {
+    "defaultMessage": "Optional"
+  },
   "lib.translations.table.column.organization": {
     "defaultMessage": "Organization"
   },
diff --git a/client/locales/ko.json b/client/locales/ko.json
index 1c765561a6d..d7c5d0af01c 100644
--- a/client/locales/ko.json
+++ b/client/locales/ko.json
@@ -6876,10 +6876,10 @@
     "defaultMessage": "닫기"
   },
   "course.userInvitations.InvitationResultDialog.duplicateInfo": {
-    "defaultMessage": "사용자가 중복 초대되었습니다. 첫 번째 사용자만 초대됩니다."
+    "defaultMessage": "사용자가 중복 초대되었습니다. 각 사용자의 첫 번째 항목만 초대됩니다."
   },
   "course.userInvitations.InvitationResultDialog.duplicateUsers": {
-    "defaultMessage": "중복 이메일 사용자 ({count})"
+    "defaultMessage": "중복 사용자 ({count})"
   },
   "course.userInvitations.InvitationResultDialog.existingCourseUsers": {
     "defaultMessage": "기존 과정 사용자 ({count})"
@@ -6902,6 +6902,12 @@
   "course.userInvitations.InvitationResultDialog.newInvitations": {
     "defaultMessage": "새 초대장 ({count})"
   },
+  "course.userInvitations.InvitationResultUsersTable.duplicateEmail": {
+    "defaultMessage": "중복 이메일"
+  },
+  "course.userInvitations.InvitationResultUsersTable.duplicateExternalId": {
+    "defaultMessage": "중복 외부 ID"
+  },
   "course.userInvitations.InvitationsBarChart.accepted": {
     "defaultMessage": "수락된 초대"
   },
@@ -6935,11 +6941,20 @@
   "course.userInvitations.InviteUsersFileUpload.failure": {
     "defaultMessage": "사용자 초대에 실패했습니다. 데이터가 올바르게 형식화되었는지 확인하세요 - {error}"
   },
+  "course.userInvitations.InviteUsersFileUpload.failureGeneric": {
+    "defaultMessage": "사용자 초대에 실패했습니다. 데이터가 올바르게 형식화되었는지 확인하세요."
+  },
   "course.userInvitations.InviteUsersFileUpload.fileUploadExample": {
-    "defaultMessage": "이름,이메일[,역할,팬텀]{br}John,test1@example.org[,학생,y]{br}Mary,test2@example.org[,티칭 어시스턴트,n]"
+    "defaultMessage": "이름,이메일,역할,팬텀,외부 ID{br}John,test1@example.org,학생,y,A0123456{br}Mary,test2@example.org,티칭 어시스턴트,n,A0123457"
   },
   "course.userInvitations.InviteUsersFileUpload.fileUploadExamplePersonalTimeline": {
-    "defaultMessage": "이름,이메일[,역할,팬텀,개인 타임라인]{br}John,test1@example.org[,학생,y,otot]{br}Mary,test2@example.org[,티칭 어시스턴트,n,fixed]"
+    "defaultMessage": "이름,이메일,역할,팬텀,개인 타임라인,외부 ID{br}John,test1@example.org,학생,y,otot,A0123456{br}Mary,test2@example.org,티칭 어시스턴트,n,fixed,A0123457"
+  },
+  "course.userInvitations.InviteUsersFileUpload.fileUploadInfoEmail": {
+    "defaultMessage": "각 초대는 과정 내에서 고유한 이메일 주소를 사용해야 합니다. 중복된 이메일은 건너뜁니다."
+  },
+  "course.userInvitations.InviteUsersFileUpload.fileUploadInfoExternalId": {
+    "defaultMessage": "외부 ID는 사용자를 외부 시스템과 연결하기 위한 선택적 필드입니다. 입력된 경우 과정 내에서 고유해야 하며, 중복된 외부 ID는 건너뜁니다."
   },
   "course.userInvitations.InviteUsersFileUpload.fileUploadInfo": {
     "defaultMessage": "다음 형식으로 .csv 파일을 업로드하세요:"
@@ -6978,7 +6993,7 @@
     "defaultMessage": "초대 삭제"
   },
   "course.userInvitations.InvitationActionButtons.resendFailure": {
-    "defaultMessage": "초대 재전송에 실패했습니다 - {error}"
+    "defaultMessage": "초대 재전송에 실패했습니다."
   },
   "course.userInvitations.InvitationActionButtons.resendSuccess": {
     "defaultMessage": "{email}로 초대 이메일을 다시 보냈습니다!"
@@ -7106,6 +7121,24 @@
   "course.users.ManageUsersTable.phantomSuccess": {
     "defaultMessage": "{name} {isPhantom, select, true {은(는) 이제 팬텀 사용자입니다} other {은(는) 이제 일반 사용자입니다}}."
   },
+  "course.users.ManageUsersTable.addIdFailure": {
+    "defaultMessage": "외부 ID를 {newId}로 설정하지 못했습니다."
+  },
+  "course.users.ManageUsersTable.addIdSuccess": {
+    "defaultMessage": "외부 ID가 {newId}로 설정되었습니다."
+  },
+  "course.users.ManageUsersTable.changeIdFailure": {
+    "defaultMessage": "ID를 {oldId}에서 {newId}로 변경하지 못했습니다."
+  },
+  "course.users.ManageUsersTable.changeIdSuccess": {
+    "defaultMessage": "ID가 {oldId}에서 {newId}로 변경되었습니다."
+  },
+  "course.users.ManageUsersTable.deleteIdFailure": {
+    "defaultMessage": "외부 ID를 삭제하지 못했습니다."
+  },
+  "course.users.ManageUsersTable.deleteIdSuccess": {
+    "defaultMessage": "외부 ID가 삭제되었습니다."
+  },
   "course.users.ManageUsersTable.renameFailure": {
     "defaultMessage": "{oldName}을(를) {newName}(으)로 이름 변경하지 못했습니다."
   },
@@ -8054,6 +8087,9 @@
   "lib.translations.table.column.email": {
     "defaultMessage": "이메일"
   },
+  "lib.translations.table.column.externalId": {
+    "defaultMessage": "외부 ID"
+  },
   "lib.translations.table.column.endAt": {
     "defaultMessage": "종료 시각"
   },
@@ -8105,6 +8141,9 @@
   "lib.translations.table.column.name": {
     "defaultMessage": "이름"
   },
+  "lib.translations.table.column.optional": {
+    "defaultMessage": "선택"
+  },
   "lib.translations.table.column.organization": {
     "defaultMessage": "조직"
   },
diff --git a/client/locales/zh.json b/client/locales/zh.json
index cf919268bb3..8cbebe4689c 100644
--- a/client/locales/zh.json
+++ b/client/locales/zh.json
@@ -6870,10 +6870,10 @@
     "defaultMessage": "关闭"
   },
   "course.userInvitations.InvitationResultDialog.duplicateInfo": {
-    "defaultMessage": "在邀请中发现重复用户。只邀请该用户第一个账号。"
+    "defaultMessage": "在邀请中发现重复用户。只邀请每个用户的第一个实例。"
   },
   "course.userInvitations.InvitationResultDialog.duplicateUsers": {
-    "defaultMessage": "具有重复电子邮件的用户（{count}）"
+    "defaultMessage": "重复用户（{count}）"
   },
   "course.userInvitations.InvitationResultDialog.existingCourseUsers": {
     "defaultMessage": "现有课程用户（{count}）"
@@ -6896,6 +6896,12 @@
   "course.userInvitations.InvitationResultDialog.newInvitations": {
     "defaultMessage": "新邀请（{count}）"
   },
+  "course.userInvitations.InvitationResultUsersTable.duplicateEmail": {
+    "defaultMessage": "重复电子邮件"
+  },
+  "course.userInvitations.InvitationResultUsersTable.duplicateExternalId": {
+    "defaultMessage": "重复外部 ID"
+  },
   "course.userInvitations.InvitationsBarChart.accepted": {
     "defaultMessage": "已接受邀请"
   },
@@ -6929,11 +6935,20 @@
   "course.userInvitations.InviteUsersFileUpload.failure": {
     "defaultMessage": "邀请用户失败。请确保你的数据格式正确 - {error}"
   },
+  "course.userInvitations.InviteUsersFileUpload.failureGeneric": {
+    "defaultMessage": "邀请用户失败。请确保你的数据格式正确。"
+  },
   "course.userInvitations.InviteUsersFileUpload.fileUploadExample": {
-    "defaultMessage": "姓名，电子邮件[,Role,Phantom]{br}John,test1@example.org[,student,y]{br}Mary,test2@example.org[,teaching_assistant,n]"
+    "defaultMessage": "姓名，电子邮件,Role,Phantom,外部编号{br}John,test1@example.org,student,y,A0123456{br}Mary,test2@example.org,teaching_assistant,n,A0123457"
   },
   "course.userInvitations.InviteUsersFileUpload.fileUploadExamplePersonalTimeline": {
-    "defaultMessage": "姓名，电子邮件[,Role,Phantom,PersonalTimeline]{br}John,test1@example.org[,student,y,otot]{br}Mary,test2@example.org[,teaching_assistant,n,fixed]"
+    "defaultMessage": "姓名，电子邮件,Role,Phantom,PersonalTimeline,外部编号{br}John,test1@example.org,student,y,otot,A0123456{br}Mary,test2@example.org,teaching_assistant,n,fixed,A0123457"
+  },
+  "course.userInvitations.InviteUsersFileUpload.fileUploadInfoEmail": {
+    "defaultMessage": "每个邀请必须使用课程内唯一的电子邮件地址。重复的电子邮件将被跳过。"
+  },
+  "course.userInvitations.InviteUsersFileUpload.fileUploadInfoExternalId": {
+    "defaultMessage": "外部编号是将用户与外部系统关联的可选字段。若提供，则必须在课程内唯一——重复的外部编号将被跳过。"
   },
   "course.userInvitations.InviteUsersFileUpload.fileUploadInfo": {
     "defaultMessage": "上传具有以下格式的 .csv 文件："
@@ -6972,7 +6987,7 @@
     "defaultMessage": "删除邀请"
   },
   "course.userInvitations.InvitationActionButtons.resendFailure": {
-    "defaultMessage": "无法重新发送邀请 - {error}"
+    "defaultMessage": "无法重新发送邀请。"
   },
   "course.userInvitations.InvitationActionButtons.resendSuccess": {
     "defaultMessage": "向 {email} 重新发送电子邮件邀请！"
@@ -7100,6 +7115,24 @@
   "course.users.ManageUsersTable.phantomSuccess": {
     "defaultMessage": "{name} {isPhantom, select, true {现在是旁听用户} other {现在是普通用户} }。"
   },
+  "course.users.ManageUsersTable.addIdFailure": {
+    "defaultMessage": "无法将外部编号设置为 {newId}"
+  },
+  "course.users.ManageUsersTable.addIdSuccess": {
+    "defaultMessage": "外部编号已设置为 {newId}"
+  },
+  "course.users.ManageUsersTable.changeIdFailure": {
+    "defaultMessage": "无法将编号从 {oldId} 更改为 {newId}"
+  },
+  "course.users.ManageUsersTable.changeIdSuccess": {
+    "defaultMessage": "编号已从 {oldId} 更改为 {newId}"
+  },
+  "course.users.ManageUsersTable.deleteIdFailure": {
+    "defaultMessage": "无法删除外部编号"
+  },
+  "course.users.ManageUsersTable.deleteIdSuccess": {
+    "defaultMessage": "外部编号已删除"
+  },
   "course.users.ManageUsersTable.renameFailure": {
     "defaultMessage": "无法将 {oldName} 重命名为 {newName}"
   },
@@ -8048,6 +8081,9 @@
   "lib.translations.table.column.email": {
     "defaultMessage": "电子邮件"
   },
+  "lib.translations.table.column.externalId": {
+    "defaultMessage": "外部编号"
+  },
   "lib.translations.table.column.endAt": {
     "defaultMessage": "结束于"
   },
@@ -8099,6 +8135,9 @@
   "lib.translations.table.column.name": {
     "defaultMessage": "姓名"
   },
+  "lib.translations.table.column.optional": {
+    "defaultMessage": "选填"
+  },
   "lib.translations.table.column.organization": {
     "defaultMessage": "组织"
   },
diff --git a/config/locales/en/activerecord/errors.yml b/config/locales/en/activerecord/errors.yml
index cb30644cca7..95f0da1660a 100644
--- a/config/locales/en/activerecord/errors.yml
+++ b/config/locales/en/activerecord/errors.yml
@@ -196,6 +196,9 @@ en:
               not_in_course: 'Selected user is not in specified course'
         course/user_invitation:
           existing_invitation: 'an open invitation exists for this email address'
+          attributes:
+            external_id:
+              taken: 'has already been taken by another user or pending invitation in this course'
         course/video:
           attributes:
             url:
@@ -213,6 +216,8 @@ en:
           deletion: 'The last tab cannot be deleted'
         course_user:
           attributes:
+            external_id:
+              taken: 'has already been taken by another user or pending invitation in this course'
             reference_timeline:
               belongs_to_course: 'must belong to course'
         duplication_traceable:
diff --git a/config/locales/en/csv.yml b/config/locales/en/csv.yml
index 020efec3c82..0d1cf158af1 100644
--- a/config/locales/en/csv.yml
+++ b/config/locales/en/csv.yml
@@ -62,3 +62,4 @@ en:
         name: 'Name'
         type: 'Student Type'
         email: 'Email'
+        external_id: 'External ID'
diff --git a/config/locales/en/errors.yml b/config/locales/en/errors.yml
index e85b2b8f03f..06ca6519b7d 100644
--- a/config/locales/en/errors.yml
+++ b/config/locales/en/errors.yml
@@ -15,8 +15,11 @@ en:
           owner: 'Owner'
           phantom: 'Phantom'
       user_invitations:
-        duplicate_user: '%{user} appears more than once in the submission.'
-        invalid_email: '%{email} is invalid: %{message}.'
+        already_enrolled: '%{email} could not be processed: already enrolled in this course.'
+        duplicate_invitation: '%{email} could not be processed: already has a pending invitation.'
+        invalid_email: '%{email} could not be processed: invalid email: %{message}.'
+        duplicate_external_id: '%{email} could not be processed: external ID "%{external_id}" is already taken.'
+        other_error: '%{email} could not be processed: %{message}.'
       user_registrations:
         invalid_code: 'You have entered an invalid invitation code.'
         code_taken: >
diff --git a/config/locales/ko/activerecord/errors.yml b/config/locales/ko/activerecord/errors.yml
index 21543f1fe1f..62c4016a70f 100644
--- a/config/locales/ko/activerecord/errors.yml
+++ b/config/locales/ko/activerecord/errors.yml
@@ -194,6 +194,9 @@ ko:
               not_in_course: '선택한 사용자가 지정된 코스에 없습니다'
         course/user_invitation:
           existing_invitation: '이 이메일 주소에는 아직 완료되지 않은 초대장이 있습니다'
+          attributes:
+            external_id:
+              taken: '이 코스에서 다른 사용자 또는 대기 중인 초대에 의해 이미 사용 중입니다'
         course/video:
           attributes:
             url:
@@ -211,6 +214,8 @@ ko:
           deletion: '마지막 탭은 삭제할 수 없습니다'
         course_user:
           attributes:
+            external_id:
+              taken: '이 코스에서 다른 사용자 또는 대기 중인 초대에 의해 이미 사용 중입니다'
             reference_timeline:
               belongs_to_course: '코스에 속해야 합니다'
         duplication_traceable:
diff --git a/config/locales/ko/csv.yml b/config/locales/ko/csv.yml
index 6a1a238665c..0a58bcd615a 100644
--- a/config/locales/ko/csv.yml
+++ b/config/locales/ko/csv.yml
@@ -62,3 +62,4 @@ ko:
         name: '이름'
         type: '학생 유형'
         email: '이메일'
+        external_id: '외부 ID'
diff --git a/config/locales/ko/errors.yml b/config/locales/ko/errors.yml
index f57ad1ceb02..ae8c0ce621e 100644
--- a/config/locales/ko/errors.yml
+++ b/config/locales/ko/errors.yml
@@ -15,8 +15,11 @@ ko:
           owner: '소유자'
           phantom: '팬텀'
       user_invitations:
-        duplicate_user: '%{user}가 제출물에 여러 번 나타납니다.'
-        invalid_email: '%{email}이(가) 유효하지 않습니다: %{message}.'
+        already_enrolled: '%{email}을(를) 처리할 수 없습니다: 이미 이 과정에 등록되어 있습니다.'
+        duplicate_invitation: '%{email}을(를) 처리할 수 없습니다: 이미 대기 중인 초대가 있습니다.'
+        invalid_email: '%{email}을(를) 처리할 수 없습니다: 유효하지 않은 이메일: %{message}.'
+        duplicate_external_id: '%{email}을(를) 처리할 수 없습니다: 외부 ID "%{external_id}"는 이미 사용 중입니다.'
+        other_error: '%{email}을(를) 처리할 수 없습니다: %{message}.'
       user_registrations:
         invalid_code: '잘못된 초대 코드를 입력하셨습니다.'
         code_taken: >
diff --git a/config/locales/zh/activerecord/errors.yml b/config/locales/zh/activerecord/errors.yml
index eb9a669b838..5cbb439debc 100644
--- a/config/locales/zh/activerecord/errors.yml
+++ b/config/locales/zh/activerecord/errors.yml
@@ -194,6 +194,9 @@ zh:
               not_in_course: '选中的用户不在指定课程中'
         course/user_invitation:
           existing_invitation: '此电子邮件地址已存在一个未完成的邀请'
+          attributes:
+            external_id:
+              taken: '该外部编号已被此课程中的其他用户或待处理邀请使用'
         course/video:
           attributes:
             url:
@@ -211,6 +214,8 @@ zh:
           deletion: '最后一项无法被删除'
         course_user:
           attributes:
+            external_id:
+              taken: '该外部编号已被此课程中的其他用户或待处理邀请使用'
             reference_timeline:
               belongs_to_course: '必须隶属于课程'
         duplication_traceable:
diff --git a/config/locales/zh/csv.yml b/config/locales/zh/csv.yml
index ac239e862fe..262bdadf441 100644
--- a/config/locales/zh/csv.yml
+++ b/config/locales/zh/csv.yml
@@ -62,3 +62,4 @@ zh:
         name: '用户名'
         type: '学生类型'
         email: 'Email'
+        external_id: '外部编号'
diff --git a/config/locales/zh/errors.yml b/config/locales/zh/errors.yml
index cb4b14af578..368e1787bb4 100644
--- a/config/locales/zh/errors.yml
+++ b/config/locales/zh/errors.yml
@@ -15,8 +15,11 @@ zh:
           owner: '拥有者'
           phantom: '虚拟用户'
       user_invitations:
-        duplicate_user: '%{user}在提交中出现了多次。'
-        invalid_email: '%{email}无效：%{message}。'
+        already_enrolled: '%{email}无法处理：已加入该课程。'
+        duplicate_invitation: '%{email}无法处理：已有待处理的邀请。'
+        invalid_email: '%{email}无法处理：邮箱无效：%{message}。'
+        duplicate_external_id: '%{email}无法处理：外部编号"%{external_id}"已被使用。'
+        other_error: '%{email}无法处理：%{message}。'
       user_registrations:
         invalid_code: '你输入了一个无效的邀请码。'
         code_taken: >
diff --git a/db/migrate/20260514052933_add_external_id_to_course_users.rb b/db/migrate/20260514052933_add_external_id_to_course_users.rb
new file mode 100644
index 00000000000..f1b039f870b
--- /dev/null
+++ b/db/migrate/20260514052933_add_external_id_to_course_users.rb
@@ -0,0 +1,5 @@
+class AddExternalIdToCourseUsers < ActiveRecord::Migration[7.2]
+  def change
+    add_column :course_users, :external_id, :string
+  end
+end
diff --git a/db/migrate/20260514100000_add_external_id_to_course_user_invitations.rb b/db/migrate/20260514100000_add_external_id_to_course_user_invitations.rb
new file mode 100644
index 00000000000..2dbab42f1f1
--- /dev/null
+++ b/db/migrate/20260514100000_add_external_id_to_course_user_invitations.rb
@@ -0,0 +1,5 @@
+class AddExternalIdToCourseUserInvitations < ActiveRecord::Migration[7.2]
+  def change
+    add_column :course_user_invitations, :external_id, :string
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 696d5837a38..41b3a10a1b9 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.2].define(version: 2026_04_06_122130) do
+ActiveRecord::Schema[7.2].define(version: 2026_05_14_100000) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
   enable_extension "uuid-ossp"
@@ -1333,6 +1333,7 @@
     t.boolean "phantom", default: false, null: false
     t.integer "timeline_algorithm"
     t.boolean "is_retryable", default: true, null: false
+    t.string "external_id"
     t.index "lower((email)::text)", name: "index_course_user_invitations_on_email"
     t.index ["confirmer_id"], name: "fk__course_user_invitations_confirmer_id"
     t.index ["course_id", "email"], name: "index_course_user_invitations_on_course_id_and_email", unique: true
@@ -1357,6 +1358,7 @@
     t.integer "timeline_algorithm", default: 0, null: false
     t.datetime "deleted_at"
     t.boolean "is_suspended", default: false, null: false
+    t.string "external_id"
     t.index ["course_id", "user_id"], name: "index_course_users_on_course_id_and_user_id", unique: true
     t.index ["course_id"], name: "fk__course_users_course_id"
     t.index ["creator_id"], name: "fk__course_users_creator_id"
diff --git a/spec/controllers/course/user_invitations_controller_spec.rb b/spec/controllers/course/user_invitations_controller_spec.rb
index 5b640aee0e7..82490bc5e5e 100644
--- a/spec/controllers/course/user_invitations_controller_spec.rb
+++ b/spec/controllers/course/user_invitations_controller_spec.rb
@@ -65,7 +65,7 @@ def replace_with_erroneous_course
           it 'sets the course errors property' do
             subject
             expect(controller.current_course.errors.count).not_to eq(0)
-            expect(controller.current_course.errors[:invitations_file].length).not_to eq(0)
+            expect(controller.current_course.errors[:base].length).not_to eq(0)
           end
         end
       end
@@ -94,7 +94,20 @@ def replace_with_erroneous_course
           replace_with_erroneous_course
           subject
           current_course = controller.current_course
-          expect(current_course.errors[:invitations_file]).not_to be_empty
+          expect(current_course.errors[:base]).not_to be_empty
+        end
+      end
+
+      context 'when a form invite has errors' do
+        subject do
+          controller.send(:propagate_errors)
+          controller
+        end
+
+        it 'propagates each error individually to :base' do
+          replace_with_erroneous_course
+          subject
+          expect(controller.current_course.errors[:base]).not_to be_empty
         end
       end
     end
@@ -116,6 +129,85 @@ def replace_with_erroneous_course
           expect(controller.send(:aggregate_errors).count).to eq(2)
         end
       end
+
+      context 'when the CSV has an external ID already taken by an existing course user' do
+        let!(:existing_user) { create(:course_student, course: course, external_id: 'EXT_DUPE') }
+        let(:invite_params) do
+          { invitations_file: fixture_file_upload('course/invitation_duplicate_external_id.csv') }
+        end
+
+        it 'returns a bad request' do
+          subject
+          expect(response).to have_http_status(:bad_request)
+        end
+
+        it 'surfaces a duplicate external ID error message' do
+          subject
+          errors = controller.send(:aggregate_errors)
+          expect(errors).not_to be_empty
+          expect(errors.any? { |e| e.include?('EXT_DUPE') }).to be(true)
+        end
+
+        it 'does not report the error as an enrolled or pending invitation error' do
+          subject
+          errors = controller.send(:aggregate_errors)
+          expect(errors.none? { |e| e.include?('already enrolled') || e.include?('pending invitation') }).
+            to be(true)
+        end
+      end
+
+      context 'when a CourseUser has an already-enrolled error' do
+        before { replace_with_erroneous_course }
+
+        it 'surfaces an already enrolled error message' do
+          errors = controller.send(:aggregate_errors)
+          expect(errors.any? { |e| e.include?('already enrolled') }).to be(true)
+        end
+      end
+
+      context 'when an invitation has a pending invitation for the same email' do
+        let(:course_with_dup_invitation) do
+          create(:course, :enrollable).tap do |c|
+            c.invitations.create!(name: 'Alice', email: 'pending@example.com')
+            c.invitations.build(name: 'Alice 2', email: 'pending@example.com')
+          end
+        end
+
+        before do
+          dup_course = course_with_dup_invitation
+          controller.define_singleton_method(:current_course) { dup_course }
+        end
+
+        it 'surfaces a duplicate invitation error message' do
+          errors = controller.send(:aggregate_errors)
+          expect(errors.any? { |e| e.include?('pending invitation') }).to be(true)
+        end
+      end
+
+      context 'when a form invite has a duplicate external ID' do
+        let!(:existing_user) { create(:course_student, course: course, external_id: 'FORM_DUPE') }
+        let(:invite_params) do
+          invitations = { generate(:nested_attribute_new_id) =>
+                          { name: generate(:name), email: generate(:email),
+                            role: :student, phantom: false, external_id: 'FORM_DUPE' } }
+          { invitations_attributes: invitations }
+        end
+
+        it 'returns a bad request' do
+          subject
+          expect(response).to have_http_status(:bad_request)
+        end
+
+        it 'returns errors as an array' do
+          subject
+          expect(response.parsed_body['errors']).to be_an(Array)
+        end
+
+        it 'includes the duplicate external ID in the error' do
+          subject
+          expect(response.parsed_body['errors'].any? { |e| e.include?('FORM_DUPE') }).to be(true)
+        end
+      end
     end
 
     describe '#resend_invitation' do
diff --git a/spec/features/course/staff_management_spec.rb b/spec/features/course/staff_management_spec.rb
index c9f13f308d7..2a058ea6c08 100644
--- a/spec/features/course/staff_management_spec.rb
+++ b/spec/features/course/staff_management_spec.rb
@@ -65,9 +65,11 @@
 
         # change name
         within find("tr.course_user_#{staff_to_change.id}") do
-          find('button.inline-edit-button', visible: false).click
-          find('input').set(new_name)
-          find('button.confirm-btn').click
+          within('.course_user_name') do
+            find('button.inline-edit-button', visible: :all).click
+            find('input').set(new_name)
+            find('button.confirm-btn').click
+          end
         end
         expect_toastify("#{staff_to_change.name} was renamed to #{new_name}")
 
diff --git a/spec/features/course/student_management_spec.rb b/spec/features/course/student_management_spec.rb
index 8a104079828..87996211e05 100644
--- a/spec/features/course/student_management_spec.rb
+++ b/spec/features/course/student_management_spec.rb
@@ -26,9 +26,11 @@
 
       # change name
       within find("tr.course_user_#{student_to_update.id}") do
-        find('button.inline-edit-button', visible: false).click
-        find('input').set(new_name)
-        find('button.confirm-btn').click
+        within('.course_user_name') do
+          find('button.inline-edit-button', visible: :all).click
+          find('input').set(new_name)
+          find('button.confirm-btn').click
+        end
       end
       expect_toastify("#{student_to_update.name} was renamed to #{new_name}")
 
diff --git a/spec/fixtures/course/invitation_duplicate_external_id.csv b/spec/fixtures/course/invitation_duplicate_external_id.csv
new file mode 100644
index 00000000000..a1ca22f4caa
--- /dev/null
+++ b/spec/fixtures/course/invitation_duplicate_external_id.csv
@@ -0,0 +1,2 @@
+name,email,role,phantom,timeline_algorithm,external_id
+Alice,alice_ext_dupe1@example.com,student,false,fixed,EXT_DUPE
diff --git a/spec/fixtures/course/invitation_external_id_wrong_header.csv b/spec/fixtures/course/invitation_external_id_wrong_header.csv
new file mode 100644
index 00000000000..5e9c2effbe0
--- /dev/null
+++ b/spec/fixtures/course/invitation_external_id_wrong_header.csv
@@ -0,0 +1,3 @@
+name,email,role,phantom,timeline_algorithm,ext_id
+Alice,alice@example.com,student,false,fixed,EXT001
+Bob,bob@example.com,teaching_assistant,false,fomo,EXT002
diff --git a/spec/fixtures/course/invitation_no_external_id_no_timeline.csv b/spec/fixtures/course/invitation_no_external_id_no_timeline.csv
new file mode 100644
index 00000000000..d4f4137ff4e
--- /dev/null
+++ b/spec/fixtures/course/invitation_no_external_id_no_timeline.csv
@@ -0,0 +1,3 @@
+name,email,role,phantom
+Alice,alice@example.com,student,false
+Bob,bob@example.com,teaching_assistant,false
diff --git a/spec/fixtures/course/invitation_with_external_id.csv b/spec/fixtures/course/invitation_with_external_id.csv
new file mode 100644
index 00000000000..acaeff99b2f
--- /dev/null
+++ b/spec/fixtures/course/invitation_with_external_id.csv
@@ -0,0 +1,4 @@
+name,email,role,phantom,timeline_algorithm,external_id
+Alice,alice@example.com,student,false,fixed,EXT001
+Bob,bob@example.com,teaching_assistant,false,fomo,EXT002
+Charlie,charlie@example.com,,,,
diff --git a/spec/fixtures/course/invitation_with_external_id_no_timeline.csv b/spec/fixtures/course/invitation_with_external_id_no_timeline.csv
new file mode 100644
index 00000000000..612e7601e6d
--- /dev/null
+++ b/spec/fixtures/course/invitation_with_external_id_no_timeline.csv
@@ -0,0 +1,4 @@
+name,email,role,phantom,external_id
+Alice,alice@example.com,student,false,EXT001
+Bob,bob@example.com,teaching_assistant,false,EXT002
+Charlie,charlie@example.com,,,
diff --git a/spec/models/course/user_invitation_spec.rb b/spec/models/course/user_invitation_spec.rb
index 0cb2fc70194..149dfc0a838 100644
--- a/spec/models/course/user_invitation_spec.rb
+++ b/spec/models/course/user_invitation_spec.rb
@@ -13,6 +13,57 @@
     let(:course) { create(:course) }
 
     describe 'validations' do
+      describe 'external_id uniqueness' do
+        let(:other_course) { create(:course) }
+
+        it 'allows multiple pending invitations with nil external_id in the same course' do
+          create(:course_user_invitation, course: course, external_id: nil)
+          invitation = build(:course_user_invitation, course: course, external_id: nil)
+          expect(invitation).to be_valid
+        end
+
+        context 'when another pending invitation in the same course has the same external_id' do
+          let!(:existing) { create(:course_user_invitation, course: course, external_id: 'dup-id') }
+
+          it 'is invalid' do
+            invitation = build(:course_user_invitation, course: course, external_id: 'dup-id')
+            expect(invitation).not_to be_valid
+            expect(invitation.errors[:external_id]).
+              to include(I18n.t('activerecord.errors.models.course/user_invitation.attributes.external_id.taken'))
+          end
+        end
+
+        context 'when a pending invitation in a different course has the same external_id' do
+          let!(:existing) { create(:course_user_invitation, course: other_course, external_id: 'some-id') }
+
+          it 'is valid' do
+            invitation = build(:course_user_invitation, course: course, external_id: 'some-id')
+            expect(invitation).to be_valid
+          end
+        end
+
+        context 'when a course user in the same course has the same external_id' do
+          let!(:existing_user) { create(:course_student, course: course, external_id: 'used-id') }
+
+          it 'is invalid' do
+            invitation = build(:course_user_invitation, course: course, external_id: 'used-id')
+            expect(invitation).not_to be_valid
+          end
+        end
+
+        context 'when only a confirmed invitation in the same course has the same external_id' do
+          let!(:confirmed) { create(:course_user_invitation, :confirmed, course: course, external_id: 'past-id') }
+
+          # The uniqueness check (UniqueExternalIdConcern) only queries unconfirmed invitations.
+          # A confirmed invitation means the user has already joined the course, so their external_id
+          # is now on the CourseUser record. The invitation row is no longer an active claim on the id.
+          it 'is valid' do
+            invitation = build(:course_user_invitation, course: course, external_id: 'past-id')
+            expect(invitation).to be_valid
+          end
+        end
+      end
+
       context 'when there is a previous invitation with the same email' do
         let(:email) { generate(:email) }
         let!(:previous_invitation) do
diff --git a/spec/models/course_user_spec.rb b/spec/models/course_user_spec.rb
index af3f13bff0c..1f61ae995ed 100644
--- a/spec/models/course_user_spec.rb
+++ b/spec/models/course_user_spec.rb
@@ -55,6 +55,67 @@
       end
     end
 
+    describe 'external_id uniqueness' do
+      let(:other_course) { create(:course, creator: owner, updater: owner) }
+
+      it 'allows multiple course users with nil external_id in the same course' do
+        create(:course_student, course: course, external_id: nil)
+        new_student = build(:course_student, course: course, external_id: nil)
+        expect(new_student).to be_valid
+      end
+
+      it 'normalizes blank external_id to nil' do
+        student = create(:course_student, course: course, external_id: '')
+        expect(student.reload.external_id).to be_nil
+      end
+
+      it 'is valid when external_id is unique in the course' do
+        student = build(:course_student, course: course, external_id: 'unique-id')
+        expect(student).to be_valid
+      end
+
+      context 'when another course user in the same course has the same external_id' do
+        let!(:existing) { create(:course_student, course: course, external_id: 'dup-id') }
+
+        it 'is invalid' do
+          student = build(:course_student, course: course, external_id: 'dup-id')
+          expect(student).not_to be_valid
+          expect(student.errors[:external_id]).
+            to include(I18n.t('activerecord.errors.models.course_user.attributes.external_id.taken'))
+        end
+      end
+
+      context 'when a course user in a different course has the same external_id' do
+        let!(:existing) { create(:course_student, course: other_course, external_id: 'some-id') }
+
+        it 'is valid' do
+          student = build(:course_student, course: course, external_id: 'some-id')
+          expect(student).to be_valid
+        end
+      end
+
+      context 'when a pending invitation in the same course has the same external_id' do
+        let!(:invitation) { create(:course_user_invitation, course: course, external_id: 'pending-id') }
+
+        it 'is invalid' do
+          student = build(:course_student, course: course, external_id: 'pending-id')
+          expect(student).not_to be_valid
+        end
+      end
+
+      context 'when only a confirmed invitation in the same course has the same external_id' do
+        let!(:invitation) { create(:course_user_invitation, :confirmed, course: course, external_id: 'confirmed-id') }
+
+        # The uniqueness check (UniqueExternalIdConcern) only queries unconfirmed invitations.
+        # A confirmed invitation means the user has already joined the course, so their external_id
+        # is now on the CourseUser record. The invitation row is no longer an active claim on the id.
+        it 'is valid' do
+          student = build(:course_student, course: course, external_id: 'confirmed-id')
+          expect(student).to be_valid
+        end
+      end
+    end
+
     describe '.staff' do
       it 'returns teaching assistant, manager and owner' do
         expect(course.course_users.staff).to contain_exactly(teaching_assistant, manager,
diff --git a/spec/models/user/email_spec.rb b/spec/models/user/email_spec.rb
index 6ee1f9e16ba..9af625a210a 100644
--- a/spec/models/user/email_spec.rb
+++ b/spec/models/user/email_spec.rb
@@ -129,6 +129,19 @@ def sign_up_on(sign_up_instance)
         end
       end
 
+      context 'when the invitation has an external_id' do
+        let!(:course) { create(:course) }
+        let!(:pending_invitation) do
+          create(:course_user_invitation, course: course, email: email_address, external_id: 'EXT-123')
+        end
+
+        it 'transfers external_id to the created CourseUser' do
+          email_record = sign_up_on(instance)
+          course_user = email_record.user.course_users.find_by(course: course)
+          expect(course_user.external_id).to eq('EXT-123')
+        end
+      end
+
       context 'when the user is already enrolled in the invited course' do
         let(:course) { create(:course) }
         let(:user) { create(:user) }
diff --git a/spec/services/course/statistics/assessment_score_summary_download_service_spec.rb b/spec/services/course/statistics/assessment_score_summary_download_service_spec.rb
index c65aad32672..d77d4891236 100644
--- a/spec/services/course/statistics/assessment_score_summary_download_service_spec.rb
+++ b/spec/services/course/statistics/assessment_score_summary_download_service_spec.rb
@@ -80,6 +80,76 @@
           expect(third_student_record[4]).to eq('')
         end
       end
+
+      context 'when students have no external_id set (backwards compatibility)' do
+        let!(:filepath) { subject }
+        let!(:csv_lines) { CSV.open(filepath, 'r').readlines }
+
+        it 'CSV has same column count as before (no external_id column)' do
+          expect(csv_lines[0].size).to eq(5)
+        end
+
+        it 'CSV header does NOT include external_id header' do
+          expect(csv_lines[0]).not_to include('external_id')
+        end
+
+        it 'existing columns (name, email, type, scores) are at same indices' do
+          header = csv_lines[0]
+          expect(header[0]).to eq(I18n.t('csv.score_summary.headers.name'))
+          expect(header[1]).to eq(I18n.t('csv.score_summary.headers.email'))
+          expect(header[2]).to eq(I18n.t('csv.score_summary.headers.type'))
+          expect(header[3]).to eq(assessment1.title)
+          expect(header[4]).to eq(assessment2.title)
+        end
+      end
+
+      context 'when some students have external_id set' do
+        let!(:student_with_ext_id) { create(:course_user, course: course, name: 'Student 4', external_id: 'EXT001') }
+        let!(:student_without_ext_id) { create(:course_user, course: course, name: 'Student 5') }
+
+        let!(:submission_a1) do
+          create(:submission, :published, assessment: assessment1, creator: student_with_ext_id.user)
+        end
+        let!(:submission_a2) do
+          create(:submission, :published, assessment: assessment2, creator: student_with_ext_id.user)
+        end
+        let!(:submission_b1) do
+          create(:submission, :published, assessment: assessment1, creator: student_without_ext_id.user)
+        end
+        let!(:submission_b2) do
+          create(:submission, :published, assessment: assessment2, creator: student_without_ext_id.user)
+        end
+
+        let(:assessment_ids_list) { [assessment1.id, assessment2.id] }
+        let(:service) { described_class.new(course, assessment_ids_list, file_name) }
+        let!(:filepath) { service.generate }
+        let!(:csv_lines) { CSV.open(filepath, 'r').readlines }
+
+        it 'CSV header includes external_id column' do
+          expect(csv_lines[0].size).to eq(6)
+          expect(csv_lines[0]).to include(I18n.t('csv.score_summary.headers.external_id'))
+        end
+
+        it 'student with external_id has it in the correct column' do
+          fourth_student_record = csv_lines[4]
+          expect(fourth_student_record[0]).to eq(student_with_ext_id.name)
+          expect(fourth_student_record[1]).to eq(student_with_ext_id.user.email)
+          expect(fourth_student_record[2]).to eq(student_with_ext_id.phantom? ? 'phantom' : 'normal')
+          expect(fourth_student_record[3]).to eq(submission_a1.grade.to_s)
+          expect(fourth_student_record[4]).to eq(submission_a2.grade.to_s)
+          expect(fourth_student_record[5]).to eq('EXT001')
+        end
+
+        it 'student without external_id has empty string in external_id column' do
+          fifth_student_record = csv_lines[5]
+          expect(fifth_student_record[0]).to eq(student_without_ext_id.name)
+          expect(fifth_student_record[1]).to eq(student_without_ext_id.user.email)
+          expect(fifth_student_record[2]).to eq(student_without_ext_id.phantom? ? 'phantom' : 'normal')
+          expect(fifth_student_record[3]).to eq(submission_b1.grade.to_s)
+          expect(fifth_student_record[4]).to eq(submission_b2.grade.to_s)
+          expect(fifth_student_record[5]).to eq('')
+        end
+      end
     end
   end
 end
diff --git a/spec/services/course/user_invitation_service_spec.rb b/spec/services/course/user_invitation_service_spec.rb
index aff5da7044e..858ecfac839 100644
--- a/spec/services/course/user_invitation_service_spec.rb
+++ b/spec/services/course/user_invitation_service_spec.rb
@@ -38,7 +38,7 @@ def temp_csv_from_attributes(records, roles = [], timeline_algorithms = [])
     let(:existing_user_attributes) do
       existing_users.each_with_index.map do |user, id|
         { name: user.name, email: user.email, phantom: false,
-          role: existing_roles[id], timeline_algorithm: existing_timeline_algorithms[id] }
+          role: existing_roles[id], timeline_algorithm: existing_timeline_algorithms[id], external_id: nil }
       end
     end
     let(:new_roles) { Course::UserInvitation.roles.keys.sample(3).map(&:to_sym) }
@@ -51,7 +51,7 @@ def temp_csv_from_attributes(records, roles = [], timeline_algorithms = [])
     let(:new_user_attributes) do
       new_users.each_with_index.map do |user, id|
         { name: user.name, email: user.email, phantom: false,
-          role: new_roles[id], timeline_algorithm: new_timeline_algorithms[id] }
+          role: new_roles[id], timeline_algorithm: new_timeline_algorithms[id], external_id: nil }
       end
     end
     let(:invalid_user_attributes) do
@@ -153,6 +153,11 @@ def invite
           expect(invite.map(&:size)).to eq([new_user_attributes.size - 1, 0, existing_user_attributes.size, 0, 1])
         end
 
+        it 'tags the duplicate user with a duplicate_email reason' do
+          _new_invitations, _existing_invitations, _new_course_users, _existing_course_users, duplicate_users = invite
+          expect(duplicate_users.first[:reason]).to eq(:duplicate_email)
+        end
+
         with_active_job_queue_adapter(:test) do
           it 'sends only one invitation to duplicate users', type: :mailer do
             expect { invite }.to change { ActionMailer::Base.deliveries.count }.
@@ -161,6 +166,180 @@ def invite
         end
       end
 
+      context 'when two invitations in the same batch share the same external_id' do
+        let(:form_attributes) do
+          { generate(:nested_attribute_new_id) => { name: 'User A', email: generate(:email),
+                                                    role: :student, phantom: false,
+                                                    external_id: 'shared-id' },
+            generate(:nested_attribute_new_id) => { name: 'User B', email: generate(:email),
+                                                    role: :student, phantom: false,
+                                                    external_id: 'shared-id' } }
+        end
+
+        it 'processes only the first and treats the second as a duplicate' do
+          result = subject.invite(form_attributes)
+          expect(result).not_to be_nil
+          new_invitations, _existing_invitations, _new_course_users, _existing_course_users, duplicate_users = result
+          expect(new_invitations.size).to eq(1)
+          expect(duplicate_users.size).to eq(1)
+          expect(duplicate_users.first[:external_id]).to eq('shared-id')
+        end
+
+        it 'tags the duplicate user with a duplicate_external_id reason' do
+          result = subject.invite(form_attributes)
+          _new_invitations, _existing_invitations, _new_course_users, _existing_course_users, duplicate_users = result
+          expect(duplicate_users.first[:reason]).to eq(:duplicate_external_id)
+        end
+      end
+
+      context 'when an invitation has a duplicate external_id matching an existing course user' do
+        let!(:existing_course_user) { create(:course_student, course: course, external_id: 'taken-id') }
+        let(:form_attributes) do
+          { generate(:nested_attribute_new_id) => { name: 'New User', email: generate(:email),
+                                                    role: :student, phantom: false,
+                                                    external_id: 'taken-id' } }
+        end
+
+        it 'returns nil' do
+          expect(subject.invite(form_attributes)).to be_nil
+        end
+      end
+
+      context 'when an invitation has a duplicate external_id matching a pending invitation' do
+        let!(:pending_invitation) { create(:course_user_invitation, course: course, external_id: 'taken-id') }
+        let(:form_attributes) do
+          { generate(:nested_attribute_new_id) => { name: 'New User', email: generate(:email),
+                                                    role: :student, phantom: false,
+                                                    external_id: 'taken-id' } }
+        end
+
+        it 'returns nil' do
+          expect(subject.invite(form_attributes)).to be_nil
+        end
+      end
+
+      context 'when a CSV (with personalized timelines) has a duplicate external_id for an existing course user' do
+        let!(:existing_course_user) { create(:course_student, course: course, external_id: 'taken-id') }
+
+        def csv_with_external_id_and_timeline(entries)
+          Tempfile.new(File.basename(__FILE__, '.*')).tap do |file|
+            file.write(CSV.generate do |csv|
+              entries.each do |entry|
+                csv << [entry[:name], entry[:email], 'student', 'false', 'fixed', entry[:external_id]]
+              end
+            end)
+            file.rewind
+          end
+        end
+
+        it 'returns nil' do
+          csv = csv_with_external_id_and_timeline(
+            [{ name: 'New User', email: generate(:email), external_id: 'taken-id' }]
+          )
+          expect(subject.invite(csv)).to be_nil
+          csv.close!
+        end
+      end
+
+      context 'when a CSV (without personalized timelines) has a duplicate external_id for an existing course user' do
+        before { course.update!(show_personalized_timeline_features: false) }
+        let!(:existing_course_user) { create(:course_student, course: course, external_id: 'taken-id') }
+
+        def csv_with_external_id_no_timeline(entries)
+          Tempfile.new(File.basename(__FILE__, '.*')).tap do |file|
+            file.write(CSV.generate do |csv|
+              entries.each do |entry|
+                csv << [entry[:name], entry[:email], 'student', 'false', entry[:external_id]]
+              end
+            end)
+            file.rewind
+          end
+        end
+
+        it 'returns nil' do
+          csv = csv_with_external_id_no_timeline(
+            [{ name: 'New User', email: generate(:email), external_id: 'taken-id' }]
+          )
+          expect(subject.invite(csv)).to be_nil
+          csv.close!
+        end
+      end
+
+      context 'CSV batch duplicate scenarios' do
+        def csv_with_timeline(entries)
+          Tempfile.new(File.basename(__FILE__, '.*')).tap do |file|
+            file.write(CSV.generate do |csv|
+              csv << ['Name', 'Email', 'Role', 'Phantom', 'Timeline', 'ExternalId']
+              entries.each do |e|
+                csv << [e[:name], e[:email], e.fetch(:role, 'student'),
+                        e.fetch(:phantom, 'false'), e.fetch(:timeline, 'fixed'), e[:external_id]]
+              end
+            end)
+            file.rewind
+          end
+        end
+
+        context 'when a CSV has two rows with the same email' do
+          it 'puts the second in duplicateUsers with reason duplicate_email' do
+            csv = csv_with_timeline([
+                                      { name: 'User A', email: 'a@example.com', external_id: 'id-1' },
+                                      { name: 'User B', email: 'a@example.com', external_id: 'id-2' }
+                                    ])
+            result = subject.invite(csv)
+            _new_invitations, _existing, _new_cu, _existing_cu, duplicate_users = result
+            expect(duplicate_users.size).to eq(1)
+            expect(duplicate_users.first[:reason]).to eq(:duplicate_email)
+            csv.close!
+          end
+        end
+
+        context 'when a CSV has two rows with the same external_id' do
+          it 'puts the second in duplicateUsers with reason duplicate_external_id' do
+            csv = csv_with_timeline([
+                                      { name: 'User A', email: 'a@example.com', external_id: 'shared-id' },
+                                      { name: 'User B', email: 'b@example.com', external_id: 'shared-id' }
+                                    ])
+            result = subject.invite(csv)
+            _new_invitations, _existing, _new_cu, _existing_cu, duplicate_users = result
+            expect(duplicate_users.size).to eq(1)
+            expect(duplicate_users.first[:reason]).to eq(:duplicate_external_id)
+            csv.close!
+          end
+        end
+
+        context 'when a CSV row has a course-duplicate email AND a batch-duplicate external_id' do
+          let(:enrolled_user) { create(:instance_user).user }
+          let!(:course_student) { create(:course_student, course: course, user: enrolled_user) }
+
+          it 'is caught as a batch external_id duplicate and does not fail the batch' do
+            csv = csv_with_timeline([
+                                      { name: 'User A', email: 'a@example.com', external_id: 'shared-id' },
+                                      { name: 'Enrolled', email: enrolled_user.email, external_id: 'shared-id' }
+                                    ])
+            result = subject.invite(csv)
+            expect(result).not_to be_nil
+            _new_invitations, _existing, _new_cu, _existing_cu, duplicate_users = result
+            expect(duplicate_users.size).to eq(1)
+            expect(duplicate_users.first[:reason]).to eq(:duplicate_external_id)
+            csv.close!
+          end
+        end
+
+        context 'when a CSV row duplicates both email and external_id within the batch' do
+          it 'is caught as a duplicate_email (email is checked first)' do
+            csv = csv_with_timeline([
+                                      { name: 'User A', email: 'a@example.com', external_id: 'id-1' },
+                                      { name: 'User B', email: 'a@example.com', external_id: 'id-1' }
+                                    ])
+            result = subject.invite(csv)
+            _new_invitations, _existing, _new_cu, _existing_cu, duplicate_users = result
+            expect(duplicate_users.size).to eq(1)
+            expect(duplicate_users.first[:reason]).to eq(:duplicate_email)
+            csv.close!
+          end
+        end
+      end
+
       context 'when an invalid email is specified' do
         let(:invalid_user_attributes) do
           [{ name: build(:user).name, email: 'xxnot an email', role: :student }]
@@ -214,15 +393,7 @@ def invite
       let(:temp_csv) { temp_csv_from_attributes(users, roles, timeline_algorithms) }
       after { temp_csv.close! }
 
-      it 'accepts a file with name/header' do
-        result = subject.send(:parse_from_file, temp_csv)
-        expect(result.length).to eq(users.length)
-      end
-
-      it 'calls #invite_users with appropriate user attributes' do
-        result = subject.send(:parse_from_file, temp_csv)
-        expect(result).to eq(user_attributes)
-      end
+      # --- file format edge cases (mode-agnostic) ---
 
       context 'when the provided file is invalid' do
         it 'raises an exception' do
@@ -243,149 +414,240 @@ def invite
         end
       end
 
-      context 'when the provided file has no roles' do
-        let(:temp_csv_without_role) { temp_csv_from_attributes(users) }
-        after { temp_csv_without_role.close! }
+      context 'when the provided file has whitespace in the fields' do
+        let(:csv_file) { file_fixture('course/invitation_whitespace.csv') }
 
-        it 'defaults the role to student' do
-          result = subject.send(:parse_from_file, temp_csv_without_role)
+        it 'strips the attributes of whitespace' do
+          result = subject.send(:parse_from_file, csv_file)
           result.each do |attr|
-            expect(attr[:role]).to eq(:student)
+            expect(attr[:name]).to eq(attr[:name].strip)
+            expect(attr[:email]).to eq(attr[:email].strip)
           end
         end
       end
 
-      context 'when the provided file has no timeline algorithm and \
-      default course timeline setting is fomo' do
-        before do
-          course.update!(default_timeline_algorithm: 'fomo')
+      context 'when the provided csv file has blanks' do
+        subject do
+          stubbed_user_invitation_service.
+            send(:parse_from_file, file_fixture('course/invitation_empty.csv'))
         end
-        let(:temp_csv_without_timeline) { temp_csv_from_attributes(users) }
-        after { temp_csv_without_timeline.close! }
 
-        it 'defaults the timeline algorithm to fomo' do
-          result = subject.send(:parse_from_file, temp_csv_without_timeline)
-          result.each do |attr|
-            expect(attr[:timeline_algorithm]).to eq('fomo')
-          end
+        it 'does not raise an exception' do
+          expect { subject }.not_to raise_exception
         end
-      end
 
-      context 'when the provided file has no timeline algorithm and \
-      default course timeline setting is stragglers' do
-        before do
-          course.update!(default_timeline_algorithm: 'stragglers')
+        it 'ignores blank entries and invites users with both name and emails or emails only' do
+          # Empty invitation CSV only has 1 full entry and 1 entry only with email
+          expect(subject.flatten.count).to eq(2)
         end
-        let(:temp_csv_without_timeline) { temp_csv_from_attributes(users) }
-        after { temp_csv_without_timeline.close! }
+      end
 
-        it 'defaults the timeline algorithm to stragglers' do
-          result = subject.send(:parse_from_file, temp_csv_without_timeline)
-          result.each do |attr|
-            expect(attr[:timeline_algorithm]).to eq('stragglers')
-          end
+      context 'when the provided csv file has no header' do
+        subject do
+          stubbed_user_invitation_service.
+            send(:parse_from_file, file_fixture('course/invitation_no_header.csv'))
         end
-      end
 
-      context 'when the provided file has no timeline algorithm and \
-      default course timeline setting is otot' do
-        before do
-          course.update!(default_timeline_algorithm: 'otot')
+        it 'does not raise an exception' do
+          expect { subject }.not_to raise_exception
         end
-        let(:temp_csv_without_timeline) { temp_csv_from_attributes(users) }
-        after { temp_csv_without_timeline.close! }
 
-        it 'defaults the timeline algorithm to otot' do
-          result = subject.send(:parse_from_file, temp_csv_without_timeline)
-          result.each do |attr|
-            expect(attr[:timeline_algorithm]).to eq('otot')
-          end
+        it 'invites all users including the first row' do
+          # No header CSV has 2 entries
+          expect(subject.flatten.count).to eq(2)
         end
       end
 
-      context 'when the provided file has whitespace in the fields' do
-        let(:csv_file) { file_fixture('course/invitation_whitespace.csv') }
+      # --- timeline-aware parsing ---
 
-        it 'strips the attributes of whitespace' do
-          result = subject.send(:parse_from_file, csv_file)
-          result.each do |attr|
-            expect(attr[:name]).to eq(attr[:name].strip)
-            expect(attr[:email]).to eq(attr[:email].strip)
-          end
-        end
-      end
+      context 'when personal timelines are enabled' do
+        before { course.update!(show_personalized_timeline_features: true) }
 
-      context 'when the csv file has slightly invalid role and/or phantom and/or \
-      timeline algorithm specifications' do
-        subject do
-          stubbed_user_invitation_service.
-            send(:parse_from_file, file_fixture('course/invitation_fuzzy_roles_phantom_timeline.csv'))
+        it 'accepts a file with name/header' do
+          result = subject.send(:parse_from_file, temp_csv)
+          expect(result.length).to eq(users.length)
         end
 
-        it 'defaults blank role column to student' do
-          expect(subject[0][:role]).to eq(:student)
+        it 'calls #invite_users with appropriate user attributes' do
+          result = subject.send(:parse_from_file, temp_csv)
+          expect(result).to eq(user_attributes)
         end
 
-        it 'defaults blank phantom to false' do
-          expect(subject[0][:phantom]).to be_falsey
-        end
+        context 'when the provided file has no roles' do
+          let(:temp_csv_without_role) { temp_csv_from_attributes(users) }
+          after { temp_csv_without_role.close! }
 
-        it 'defaults blank timeline algorithm to course default (fixed)' do
-          expect(subject[0][:timeline_algorithm]).to eq('fixed')
+          it 'defaults the role to student' do
+            result = subject.send(:parse_from_file, temp_csv_without_role)
+            result.each do |attr|
+              expect(attr[:role]).to eq(:student)
+            end
+          end
         end
 
-        it 'parses roles correctly anyway' do
-          expect(subject[1][:role]).to eq(:teaching_assistant)
-          expect(subject[2][:role]).to eq(:manager)
-          expect(subject[3][:role]).to eq(:owner)
-          expect(subject[4][:role]).to eq(:observer)
-          expect(subject[5][:role]).to eq(:teaching_assistant)
-        end
+        context 'when the csv file has slightly invalid role/phantom/timeline algorithm specifications' do
+          subject do
+            stubbed_user_invitation_service.
+              send(:parse_from_file, file_fixture('course/invitation_fuzzy_roles_phantom_timeline.csv'))
+          end
+
+          it 'defaults blank role column to student' do
+            expect(subject[0][:role]).to eq(:student)
+          end
+
+          it 'defaults blank phantom to false' do
+            expect(subject[0][:phantom]).to be_falsey
+          end
+
+          it 'defaults blank timeline algorithm to course default (fixed)' do
+            expect(subject[0][:timeline_algorithm]).to eq('fixed')
+          end
 
-        it 'parses phantom columns correctly anyway' do
-          expect(subject[1][:phantom]).to be_falsey
-          (6..8).each do |i|
-            expect(subject[i][:phantom]).to be_truthy
+          it 'parses roles correctly anyway' do
+            expect(subject[1][:role]).to eq(:teaching_assistant)
+            expect(subject[2][:role]).to eq(:manager)
+            expect(subject[3][:role]).to eq(:owner)
+            expect(subject[4][:role]).to eq(:observer)
+            expect(subject[5][:role]).to eq(:teaching_assistant)
+          end
+
+          it 'parses phantom columns correctly anyway' do
+            expect(subject[1][:phantom]).to be_falsey
+            (6..8).each do |i|
+              expect(subject[i][:phantom]).to be_truthy
+            end
+          end
+
+          it 'parses timeline algorithms correctly anyway' do
+            expect(subject[1][:timeline_algorithm]).to eq(:stragglers)
+            expect(subject[2][:timeline_algorithm]).to eq(:otot)
+            expect(subject[3][:timeline_algorithm]).to eq(:fomo)
+            expect(subject[4][:timeline_algorithm]).to eq(:fixed)
           end
         end
 
-        it 'parses roles correctly anyway' do
-          expect(subject[1][:timeline_algorithm]).to eq(:stragglers)
-          expect(subject[2][:timeline_algorithm]).to eq(:otot)
-          expect(subject[3][:timeline_algorithm]).to eq(:fomo)
-          expect(subject[4][:timeline_algorithm]).to eq(:fixed)
+        context 'when no timeline algorithm column is present' do
+          let(:temp_csv_without_timeline) { temp_csv_from_attributes(users) }
+          after { temp_csv_without_timeline.close! }
+
+          context 'when the course default is fomo' do
+            before { course.update!(default_timeline_algorithm: 'fomo') }
+
+            it 'defaults the timeline algorithm to fomo' do
+              result = subject.send(:parse_from_file, temp_csv_without_timeline)
+              result.each do |attr|
+                expect(attr[:timeline_algorithm]).to eq('fomo')
+              end
+            end
+          end
+
+          context 'when the course default is stragglers' do
+            before { course.update!(default_timeline_algorithm: 'stragglers') }
+
+            it 'defaults the timeline algorithm to stragglers' do
+              result = subject.send(:parse_from_file, temp_csv_without_timeline)
+              result.each do |attr|
+                expect(attr[:timeline_algorithm]).to eq('stragglers')
+              end
+            end
+          end
+
+          context 'when the course default is otot' do
+            before { course.update!(default_timeline_algorithm: 'otot') }
+
+            it 'defaults the timeline algorithm to otot' do
+              result = subject.send(:parse_from_file, temp_csv_without_timeline)
+              result.each do |attr|
+                expect(attr[:timeline_algorithm]).to eq('otot')
+              end
+            end
+          end
         end
-      end
 
-      context 'when the provided csv file has blanks' do
-        subject do
-          stubbed_user_invitation_service.
-            send(:parse_from_file, file_fixture('course/invitation_empty.csv'))
+        context 'when the csv has an external_id column' do
+          subject do
+            stubbed_user_invitation_service.
+              send(:parse_from_file, file_fixture('course/invitation_with_external_id.csv'))
+          end
+
+          it 'parses external_id from col 6 correctly' do
+            expect(subject[0][:external_id]).to eq('EXT001')
+            expect(subject[1][:external_id]).to eq('EXT002')
+          end
+
+          it 'sets external_id to nil when blank' do
+            expect(subject[2][:external_id]).to be_nil
+          end
         end
 
-        it 'does not raise an exception' do
-          expect { subject }.not_to raise_exception
+        context 'when the csv has no external_id column' do
+          let(:csv_without_external_id) { file_fixture('course/invitation_fuzzy_roles_phantom_timeline.csv') }
+
+          it 'sets external_id to nil for all rows' do
+            result = stubbed_user_invitation_service.send(:parse_from_file, csv_without_external_id)
+            result.each do |attr|
+              expect(attr[:external_id]).to be_nil
+            end
+          end
         end
 
-        it 'ignores blank entries and invites users with both name and emails or emails only' do
-          # Empty invitation CSV only has 1 full entry and 1 entry only with email
-          expect(subject.flatten.count).to eq(2)
+        context 'when the csv header uses a slightly wrong external_id column name' do
+          subject do
+            stubbed_user_invitation_service.
+              send(:parse_from_file, file_fixture('course/invitation_external_id_wrong_header.csv'))
+          end
+
+          it 'still detects and skips the header row' do
+            expect(subject.length).to eq(2)
+          end
+
+          it 'still parses external_id from col 6 correctly' do
+            expect(subject[0][:external_id]).to eq('EXT001')
+            expect(subject[1][:external_id]).to eq('EXT002')
+          end
         end
       end
 
-      context 'when the provided csv file has no header' do
-        subject do
-          stubbed_user_invitation_service.
-            send(:parse_from_file, file_fixture('course/invitation_no_header.csv'))
-        end
+      context 'when personal timelines are disabled' do
+        before { course.update!(show_personalized_timeline_features: false) }
 
-        it 'does not raise an exception' do
-          expect { subject }.not_to raise_exception
+        context 'when the csv has an external_id column' do
+          subject do
+            stubbed_user_invitation_service.
+              send(:parse_from_file, file_fixture('course/invitation_with_external_id_no_timeline.csv'))
+          end
+
+          it 'parses external_id from col 5 correctly' do
+            expect(subject[0][:external_id]).to eq('EXT001')
+            expect(subject[1][:external_id]).to eq('EXT002')
+          end
+
+          it 'sets external_id to nil when blank' do
+            expect(subject[2][:external_id]).to be_nil
+          end
+
+          it 'auto-fills timeline_algorithm with course default' do
+            result = stubbed_user_invitation_service.send(
+              :parse_from_file,
+              file_fixture('course/invitation_with_external_id_no_timeline.csv')
+            )
+            result.each do |attr|
+              expect(attr[:timeline_algorithm]).to eq(course.default_timeline_algorithm)
+            end
+          end
         end
 
-        it 'invites all users including the first row' do
-          # No header CSV has 2 entries
-          expect(subject.flatten.count).to eq(2)
+        context 'when the csv has no external_id column' do
+          subject do
+            stubbed_user_invitation_service.
+              send(:parse_from_file, file_fixture('course/invitation_no_external_id_no_timeline.csv'))
+          end
+
+          it 'sets external_id to nil for all rows' do
+            subject.each do |attr|
+              expect(attr[:external_id]).to be_nil
+            end
+          end
         end
       end
     end