diff --git a/app/assets/javascripts/filtered_search/dropdown_user.js b/app/assets/javascripts/filtered_search/dropdown_user.js
index 7246ccbb281..720fbc87ea0 100644
--- a/app/assets/javascripts/filtered_search/dropdown_user.js
+++ b/app/assets/javascripts/filtered_search/dropdown_user.js
@@ -15,6 +15,7 @@ class DropdownUser extends gl.FilteredSearchDropdown {
         params: {
           per_page: 20,
           active: true,
+          group_id: this.getGroupId(),
           project_id: this.getProjectId(),
           current_user: true,
         },
@@ -47,6 +48,10 @@ class DropdownUser extends gl.FilteredSearchDropdown {
     super.renderContent(forceShowList);
   }
 
+  getGroupId() {
+    return this.input.getAttribute('data-group-id');
+  }
+
   getProjectId() {
     return this.input.getAttribute('data-project-id');
   }
diff --git a/app/views/shared/issuable/_filter.html.haml b/app/views/shared/issuable/_filter.html.haml
index c4ed7f6e750..d3f0aa2d339 100644
--- a/app/views/shared/issuable/_filter.html.haml
+++ b/app/views/shared/issuable/_filter.html.haml
@@ -11,13 +11,13 @@
           - if params[:author_id].present?
             = hidden_field_tag(:author_id, params[:author_id])
           = dropdown_tag(user_dropdown_label(params[:author_id], "Author"), options: { toggle_class: "js-user-search js-filter-submit js-author-search", title: "Filter by author", filter: true, dropdown_class: "dropdown-menu-user dropdown-menu-selectable dropdown-menu-author js-filter-submit",
-            placeholder: "Search authors", data: { any_user: "Any Author", first_user: current_user.try(:username), current_user: true, project_id: @project.try(:id), selected: params[:author_id], field_name: "author_id", default_label: "Author" } })
+            placeholder: "Search authors", data: { any_user: "Any Author", first_user: current_user&.username, current_user: true, project_id: @project&.id, group_id: @group&.id, selected: params[:author_id], field_name: "author_id", default_label: "Author" } })
 
         .filter-item.inline
           - if params[:assignee_id].present?
             = hidden_field_tag(:assignee_id, params[:assignee_id])
           = dropdown_tag(user_dropdown_label(params[:assignee_id], "Assignee"), options: { toggle_class: "js-user-search js-filter-submit js-assignee-search", title: "Filter by assignee", filter: true, dropdown_class: "dropdown-menu-user dropdown-menu-selectable dropdown-menu-assignee js-filter-submit",
-            placeholder: "Search assignee", data: { any_user: "Any Assignee", first_user: current_user.try(:username), null_user: true, current_user: true, project_id: @project.try(:id), group_id: @group&.id, selected: params[:assignee_id], field_name: "assignee_id", default_label: "Assignee" } })
+            placeholder: "Search assignee", data: { any_user: "Any Assignee", first_user: current_user&.username, null_user: true, current_user: true, project_id: @project&.id, group_id: @group&.id, selected: params[:assignee_id], field_name: "assignee_id", default_label: "Assignee" } })
 
         .filter-item.inline.milestone-filter
           = render "shared/issuable/milestone_dropdown", selected: finder.milestones.try(:first), name: :milestone_title, show_any: true, show_upcoming: true, show_started: true
diff --git a/changelogs/unreleased/20824-scope-users-to-members-in-group-issuable-list.yml b/changelogs/unreleased/20824-scope-users-to-members-in-group-issuable-list.yml
new file mode 100644
index 00000000000..245b8129de8
--- /dev/null
+++ b/changelogs/unreleased/20824-scope-users-to-members-in-group-issuable-list.yml
@@ -0,0 +1,5 @@
+---
+title: Return only group's members in user dropdowns on issuables list pages
+merge_request: 14249
+author:
+type: changed
diff --git a/spec/features/groups/user_sees_users_dropdowns_in_issuables_list_spec.rb b/spec/features/groups/user_sees_users_dropdowns_in_issuables_list_spec.rb
new file mode 100644
index 00000000000..5ed4f3ad2bc
--- /dev/null
+++ b/spec/features/groups/user_sees_users_dropdowns_in_issuables_list_spec.rb
@@ -0,0 +1,22 @@
+require 'spec_helper'
+
+feature 'Groups > User sees users dropdowns in issuables list' do
+  let(:entity) { create(:group) }
+  let(:user_in_dropdown) { create(:user) }
+  let!(:user_not_in_dropdown) { create(:user) }
+  let!(:project) { create(:project, group: entity) }
+
+  before do
+    entity.add_developer(user_in_dropdown)
+  end
+
+  it_behaves_like 'issuable user dropdown behaviors' do
+    let(:issuable) { create(:issue, project: project) }
+    let(:issuables_path) { issues_group_path(entity) }
+  end
+
+  it_behaves_like 'issuable user dropdown behaviors' do
+    let(:issuable) { create(:merge_request, source_project: project) }
+    let(:issuables_path) { merge_requests_group_path(entity) }
+  end
+end
diff --git a/spec/javascripts/filtered_search/dropdown_user_spec.js b/spec/javascripts/filtered_search/dropdown_user_spec.js
index b3c9bca64cc..02415485d19 100644
--- a/spec/javascripts/filtered_search/dropdown_user_spec.js
+++ b/spec/javascripts/filtered_search/dropdown_user_spec.js
@@ -10,6 +10,7 @@ describe('Dropdown User', () => {
     beforeEach(() => {
       spyOn(gl.DropdownUser.prototype, 'bindEvents').and.callFake(() => {});
       spyOn(gl.DropdownUser.prototype, 'getProjectId').and.callFake(() => {});
+      spyOn(gl.DropdownUser.prototype, 'getGroupId').and.callFake(() => {});
       spyOn(gl.DropdownUtils, 'getSearchInput').and.callFake(() => {});
 
       dropdownUser = new gl.DropdownUser({
@@ -38,6 +39,7 @@ describe('Dropdown User', () => {
     beforeEach(() => {
       spyOn(gl.DropdownUser.prototype, 'bindEvents').and.callFake(() => {});
       spyOn(gl.DropdownUser.prototype, 'getProjectId').and.callFake(() => {});
+      spyOn(gl.DropdownUser.prototype, 'getGroupId').and.callFake(() => {});
     });
 
     it('should return endpoint', () => {
diff --git a/spec/support/shared_examples/features/issuables_user_dropdown_behaviors_shared_examples.rb b/spec/support/shared_examples/features/issuables_user_dropdown_behaviors_shared_examples.rb
new file mode 100644
index 00000000000..c92c7f603d6
--- /dev/null
+++ b/spec/support/shared_examples/features/issuables_user_dropdown_behaviors_shared_examples.rb
@@ -0,0 +1,21 @@
+shared_examples 'issuable user dropdown behaviors' do
+  include FilteredSearchHelpers
+
+  before do
+    issuable # ensure we have at least one issuable
+    sign_in(user_in_dropdown)
+  end
+
+  %w[author assignee].each do |dropdown|
+    describe "#{dropdown} dropdown", :js do
+      it 'only includes members of the project/group' do
+        visit issuables_path
+
+        filtered_search.set("#{dropdown}:")
+
+        expect(find("#js-dropdown-#{dropdown} .filter-dropdown")).to have_content(user_in_dropdown.name)
+        expect(find("#js-dropdown-#{dropdown} .filter-dropdown")).not_to have_content(user_not_in_dropdown.name)
+      end
+    end
+  end
+end
diff --git a/spec/support/project_features_apply_to_issuables_shared_examples.rb b/spec/support/shared_examples/features/project_features_apply_to_issuables_shared_examples.rb
similarity index 100%
rename from spec/support/project_features_apply_to_issuables_shared_examples.rb
rename to spec/support/shared_examples/features/project_features_apply_to_issuables_shared_examples.rb