BuddyPress.org

It would be very easy to modify your forum code so that only registered members can view posts. You could say “if the current user is not logged in, display a ‘sorry’ message, else proceed with setting up and displaying the forum posts. You could even set it up so they could view a few teaser posts at the top, and then display the ‘you must login’ message. I may in fact try this out on a few groups!