Option to filter add member autocomplete for private repositories

Problem to solve

Adding members to a private repository is time consuming and error prone, since GitLab autocompletes the entire public set of users when searching by name or username. Unless you spell their username exactly, this makes it easy to accidentally add the wrong person to a private repository as multiple people share the same name. If you do not recall their username, it is time consuming to go back and find a repository to which they already have access in order to look it up.

Intended users

Anyone with private repositories on gitlab.com.

Proposal

Have an option to restrict the add member autocomplete to members that are already added to existing repositories in the organization, possibly defaulted on for private repositories.

Further details

Benefits: It becomes easier to add users without having to go back to an existing repository to look up their username and copy / paste it exactly. Security is improved as it is less likely to add the wrong users to a private repository and expose confidential source code.

Permissions and Security

No changes.

What does success look like, and how can we measure that?

The add user dropdown doesn't autocomplete every user on gitlab.com when the feature is enabled.

Links / references

Attached is a screenshot showing the current behavior.