You can do this already, can't you?
Everyone already belongs to the User group, create another group and add whoever you want to it. You can set the priority of that group so it's displayed just like an Administer's rank is displayed. You can pick whichever color you want for your new group in the User Ranks section of your ACP.
Am I missing something?
I'm still new at working this out lol and so what's an good priority number then? lets say they in there main group "A" and I added them into group "B" and so what's the priority for group B number