BuddyPress.org

Thank you Jose and Patrick for the kind words. Since I checked the forum here today and to my surprise found the topic I thought I would clarify.
@migraine. no, this plugin does not support categories inside group forum. It rather allows a separate forum(but coupled with BuddyPress), so It is not what you are looking for. It allows to have independent forum/subforums etc, but does not add multiple forum within the group.