A user group promotion adds them into an additional secondary group of your choosing. It's not really that convoluted. User groups are great for permissions, of course, but also for things like user group styling and banners. It also means you can easily do a search to all members in a certain group, or send an email/alert to users within a certain group. When creating the user group you just leave all permissions set to default, so being added to an additional secondary group shouldn't affect anything you have set up permissions wise, or for staff. Staff will still be promoted into the user group too or you can set up the criteria so they won't.
Lots of options there, but at a basic level it does what you want.