MyBB Community Forums

I would like a plugin or code that I can apply so that only a certain group can join a made group. Like for instance the premium group is the group you can to be in so you can join the "Cool" group freely but registered users can't join the "cool" group. If there is a plugin that can play off of the feature in mybb for users to join and leave groups freely please tell me the name. If there is a code edit can you please link me to a tutorial or post a tutorial.
Thank in advanced.