This Plugin IS NO LONGER MAINTAINED
use https://wordpress.org/plugins/bbp-private-groups/
This plugin displays private forums titles to non logged-in users in main index.
This allows non logged-in users to see that forums exist, but have no acces to these.
If selected by a non logged-in user, the use is sent to a page (determined in settings), allowing the admin to determine what information is given, eg login, registration, text etc.
It also has the options to
display forum content (descriptions) for sub forums
remove the prefix 'private' from all private forums
Works with bbpress 2.5.4