All users are visible in @mentions autocomplete list

This comes up from time to time. There isn’t a means to hide groups of users from other users, just their group membership.

If you don’t want users to know that other users exist then you need to disable access to user profiles. I don’t think that there’s an easy way to do that.

There is this site setting allow users to hide profile, but that just allows users to hide themselves. I think you need a plugin to enable it for every user.