The new bbPress standalone plugin for WP is great. It makes sense for this to be implemented as a plugin, using custom post types as it does. One limitation to the current version of the plugin is that forums can be either public to all or private to all (based on role). If you don’t want to get into the mess of defining roles for your users to manage different forums or if you are already using Paid Memberships Pro, below is a good solution for locking down bbPress forums based on PMPro membership levels.
To get this to work, you must have your forums and membership levels created. The forums should be set as “public” in bbPress. Then update the $restricted_forums array in the code below and place it in your functions.php or somewhere else where it will be run. Enjoy!