[Kinneas]

You can create Usergroups which have permission to access the special forums. Provided you set the permissions correctly, you can stop anyone but people in that usergroup from seeing the special forum.

You can either manually add users to the usergroup, or make it a joinable group. If you make it a joinable group, you can either make it open access (Anyone can join or leave at any time) or based on approvals (You would set up a group leader, doesn't have to be yourself, who approves or rejects applications to join that usergroup).

That's one example of how you could achieve what you want. There are other ways of doing it, too.