Ability to filter GitLab users when adding members to a project

Proposal

When adding members to a repository (GitLab.com) per the official documentation, the autocomplete feature currently only supports searching for users via username, name (first last) or specific email address and searches all GitLab users. There is currently no way to limit the search to users with a specific email domain (*@google.com) or namespace (group).

For larger customers with many (~500) GitLab users and open source contributors, there can be a requirement to limit access to certain projects to only company employees that use the company email domain. This can be an expensive task to search and filter through each user and confirm who needs to be added to a project. Adding an email domain search functionality would ease this task and also add another security layer, as only users with the required email domain (@google.com) would be added as members and avoid a potential data breach if a private user was added to an internal project. Similarly, the ability to limit the results to the top level namespace members would be useful.

I also found this Issue which also highlights challenges with this feature and has proposed solutions, but none cover this particular use case (searchable by email domain).