Fix Error 500 when inviting user already present

A project admin attempting to invite an already-invited user
will see a confusing Error 500 message.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/61574

app/services/members/create_service.rb

@@ -23,7 +23,16 @@ module Members
       members.each do |member|
         if member.errors.any?
-          errors << "#{member.user.username}: #{member.errors.full_messages.to_sentence}"
+          current_error =
+            # Invited users may not have an associated user
+            if member.user.present?
+              "#{member.user.username}: "
+            else
+              ""
+            end
+
+          current_error += member.errors.full_messages.to_sentence
+          errors << current_error
         else
           after_execute(member: member)
         end

changelogs/unreleased/sh-fix-invited-members.yml

+---
+title: Fix Error 500 when inviting user already present
+merge_request: 28198
+author:
+type: fixed

spec/services/members/create_service_spec.rb

@@ -44,7 +44,18 @@ describe Members::CreateService do
     result = described_class.new(user, params).execute(project)
     expect(result[:status]).to eq(:error)
-    expect(result[:message]).to include(project_user.username)
+    expect(result[:message]).to include("#{project_user.username}: Access level is not included in the list")
     expect(project.users).not_to include project_user
   end
+
+  it 'does not add a member with an existing invite' do
+    invited_member = create(:project_member, :invited, project: project)
+
+    params = { user_ids: invited_member.invite_email,
+               access_level: Gitlab::Access::GUEST }
+    result = described_class.new(user, params).execute(project)
+
+    expect(result[:status]).to eq(:error)
+    expect(result[:message]).to eq('Invite email has already been taken')
+  end
 end