It's all in the stock UBB...
Go Forum Settings, click Details on the private forum. Click on the "Restrict Viewing and Writing of Posts To Admins, Moderators and other authorized* users" checkbox to enable that. Then on posting restrictions, make it Admins/Mods/Authorized for both post topics/replies. Now go to View/Edit Member Records. Choose the private forum under "Access to Private Forums" and hit Retrieve Members Now. You should see a whole list of who has access. If you want to give someone access or don't give them access anymore, just search for their member number and enter their profile. Check/Uncheck the boxes on the bottom the profiles to give or take away access.
