Add endpoint for activating an waiting member

Allows activating/approving a pending/awaiting member for a group. These
members are in this state when the root group's user cap has been
reached

Changelog: added
EE: true

ee/lib/ee/api/members.rb

@@ -54,6 +54,28 @@ module EE
             end
           end
 
+          desc 'Approves a pending member'
+          params do
+            requires :user_id, type: Integer, desc: 'The user ID of the member requiring approval'
+          end
+          put ':id/members/:user_id/approve' do
+            group = find_group!(params[:id])
+            user = ::User.find(params[:user_id])
+
+            bad_request! unless group.root?
+            bad_request! unless can?(current_user, :admin_group_member, group)
+
+            result = ::Members::ActivateService
+              .new(group, user: user, current_user: current_user)
+              .execute
+
+            if result[:status] == :success
+              no_content!
+            else
+              bad_request!(result[:message])
+            end
+          end
+
           desc 'Gets a list of billable users of root group.' do
             success Entities::Member
           end

ee/spec/requests/api/members_spec.rb

@@ -947,4 +947,105 @@ RSpec.describe API::Members do
       end
     end
   end
+
+  context 'group with pending members' do
+    let_it_be(:owner) { create(:user, username: 'owner_user') }
+    let_it_be(:developer) { create(:user) }
+    let_it_be(:group) { create(:group, :public) }
+    let_it_be(:subgroup) { create(:group, parent: group) }
+    let_it_be(:project) { create(:project, group: group) }
+    let_it_be(:not_an_owner) { create(:user) }
+
+    let(:url) { "/groups/#{group.id}/members/#{developer.id}/approve" }
+
+    before do
+      group.add_owner(owner)
+    end
+
+    describe 'PUT /groups/:id/members/:user_id/approve' do
+      context 'with invalid params' do
+        context 'when a subgroup is used' do
+          let(:url) { "/groups/#{subgroup.id}/members/#{developer.id}/approve" }
+
+          it 'returns a bad request response' do
+            put api(url, not_an_owner)
+
+            expect(response).to have_gitlab_http_status(:bad_request)
+          end
+        end
+
+        context 'when no group is found' do
+          let(:url) { "/groups/#{non_existing_record_id}/members/#{developer.id}/approve" }
+
+          it 'returns a not found response' do
+            put api(url, owner)
+
+            expect(response).to have_gitlab_http_status(:not_found)
+          end
+        end
+      end
+
+      context 'when the current user does not have the :admin_group_member ability' do
+        it 'returns a bad request response' do
+          put api(url, not_an_owner)
+
+          expect(response).to have_gitlab_http_status(:bad_request)
+        end
+      end
+
+      context 'when the current user has permission to approve' do
+        context 'when the user is not found' do
+          let(:url) { "/groups/#{group.id}/members/#{non_existing_record_id}/approve" }
+
+          it 'returns not found response' do
+            put api(url, owner)
+
+            expect(response).to have_gitlab_http_status(:not_found)
+          end
+        end
+
+        context 'when the activation fails due to no members to activate' do
+          it 'returns a bad request response' do
+            put api(url, owner)
+
+            expect(response).to have_gitlab_http_status(:bad_request)
+          end
+        end
+
+        context 'when the user is a pending member of a root group' do
+          it 'activates the member' do
+            pending_member = create(:group_member, :awaiting, group: group, user: developer)
+            put api(url, owner)
+
+            expect(response).to have_gitlab_http_status(:success)
+            expect(pending_member.reload.active?).to be true
+          end
+        end
+
+        context 'when the user is a pending member of a subgroup' do
+          it 'activates the member' do
+            pending_member = create(:group_member, :awaiting, group: subgroup, user: developer)
+
+            put api(url, owner)
+
+            expect(response).to have_gitlab_http_status(:success)
+            expect(pending_member.reload.active?).to be true
+          end
+        end
+
+        context 'when the user is a pending member of a project' do
+          let(:developer) { create(:user) }
+
+          it 'activates the member' do
+            pending_member = create(:project_member, :awaiting, project: project, user: developer)
+
+            put api(url, owner)
+
+            expect(response).to have_gitlab_http_status(:success)
+            expect(pending_member.reload.active?).to be true
+          end
+        end
+      end
+    end
+  end
 end