To define private forums, all you need to do is to open the bb_specials.php file and insert the values (forum ID's and user ids), like below in bold, and save/upload the file back to the server.
8- is forum ID you want to make private
4, 65, 75, 366 - user_ids (users) which will have access to forum 8
$clForumsUsers=array(4, 65, 75, 366);
Again, all such things are logically described in the manual or you can use the search function to find even more examples.