How to allow someone to add users to a private forum
 

I want to have a private forum, where one user controls access to an area who is not an admin, but a mod of that message area.

I was thinking if I made the access available to a group, and allowed people to request joining the group, and the mod of that group could allow people to join and then it would give them access to see the messages in that one area.

I'm just not sure how to create the proper permissions so that anyone outside of the group could not see the messages in this area.

I hope I said that all correctly.