bbPress.org

bbpress uses the default wordpress user setup, but adds a bbpres role to the wp-capabilities in user_meta.

So as long as there is a wp_user then on first login by that user bbpress will allocate the default bbpress role as set in Dashboard>settings>forums.

In think this plugin might be your solution, as it seems to create and update wordpress via AD

https://wordpress.org/plugins/active-directory-integration/

and then bbpress will use that wordpress database to add the role.