There needs to be a way to use the Users panel to add one or multiple users to an existing group. The only way to do this is to go to Users > Groups and manually type in the user's name or add a rule. If you go to Users panel and check one user, you cannot Add Group. If you select 2 or more users, you can Add Group but only lets you create a new group. If you name it the same as an existing group, it just has two groups of the same name and different users.