Bulk adding members needs to consider user records as well as the ids

What does this MR do and why?

allows users that are sent to the bulk add_users method to be found in existing users if they are already members

this allows us to not try and create another member(which would fail) and instead use the existing member record that is found.

currently this is only called from

• ProjectMember#add_users_to_projects
• ProjectTeam#add_users
• Group#add_users

which do not seem to have uses currently where users are purely sent in as a record(they use user.id as array), they could because the code allows it, so we should prevent a possible future bug.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #352617 (closed)