Check if user is an int before passing to find_by_id

app/models/member.rb

@@ -257,7 +257,9 @@ class Member < ApplicationRecord
     def retrieve_user(user)
       return user if user.is_a?(User)
 
-      User.find_by(id: user) || User.find_by(email: user) || user
+      return User.find_by(id: user) if user.is_a?(Integer)
+
+      User.find_by(email: user) || user
     end
 
     def retrieve_member(source, user, existing_members)

changelogs/unreleased/handle_inviting_members_whose_emails_start_with_numbers.yml

+---
+title: Fix bug inviting members whose emails start with numbers
+merge_request: 27848
+author: Lee Tickett
+type: fixed

spec/models/member_spec.rb

@@ -342,6 +342,17 @@ describe Member do
               expect(source.members.invite.pluck(:invite_email)).to include('user@example.com')
             end
           end
+
+          context 'when called with an unknown user email starting with a number' do
+            it 'creates an invited member', :aggregate_failures do
+              email_starting_with_number = "#{user.id}_email@example.com"
+
+              described_class.add_user(source, email_starting_with_number, :maintainer)
+
+              expect(source.members.invite.pluck(:invite_email)).to include(email_starting_with_number)
+              expect(source.users.reload).not_to include(user)
+            end
+          end
         end
 
         context 'when current_user can update member' do