All users are public and therefore visible to any other user
When going to the main issue page with all the users issues, the dropdown menus "Author" and "Assignee" show more entries than necessary.
Viewing the issue page for a specific project or group, only the participating users are listed.
Therefore the main issue page should only list the sum of all the users in the projects and groups of the user.
In general it should be possible to restrict interfaces where a complete list of users can be obtained (API, etc.) to only show a subset of users, that share project or group membership with the current user.
A company running GitLab for their customers usually does not want to publish a complete list of all users to any other user, as this could leak information about internal business relations.