BuddyPress.org

Hmm it depends how you want your site to work. At the moment, it’s a one group to forum mapping. That means, for each forum (or sub-forum), you’re going to need a group.

How many (sub)forums on your existing site? If you have more forums than you want BuddyPress Groups created, we need to do some more work to configure your setup to integrate your WordPress into an external bbPress installation. This is not technically easy, and would involve you creating a separate theme for the bbPress site itself.
A benefit of this approach is that you have full access to a traditional forum look-and-feel (bbPress’).

If, however, you’re happy with how the one forum to group mapping works with BuddyPress, you’ve already done the installation (i.e. clicked the button that said “use an existing bbPress installation”). What you’ll need to do next is move your users from the bb_users table into wp_users (and usermeta too if you want, but be careful and consider if you need to do that), create some groups and then edit their database entries to point to the existing bbPress tables.