It would be nice if we could have a folder structure in User Manager to manage users and groups.
Groups can contain users, but there is only one level of "grouping". You can't have a group in a group, etc. We did this in CrushFTP v4, and decided against it for performance reasons since username are always required to be unique anyway.
So its a flat listing of group names, and a flat listing of all users, or the limited list of users that are part of a group.