bbp_forum_recursive_group_id( $forum_id )

function bbp_forum_recursive_group_id( $forum_id ) {

	if ( empty( $forum_id ) ) {
		return false;
	}

	// initialize a few things.
	$group_id          = 0;
	$found_group_forum = false;
	$reached_the_top   = false;

	// This loop works our way up to the top of the topic->sub-forum->parent-forum hierarchy..
	// We will stop climbing when we find a forum_id that is also the id of a group's forum..
	// When we find that, we've found the group, and we can stop looking..
	// Or if we get to the top of the hierarchy, we'll bail out of the loop, never having found a forum.
	// that is associated with a group..
	while ( ! $found_group_forum && ! $reached_the_top ) {
		$forum_group_ids = bbp_get_forum_group_ids( $forum_id );
		if ( ! empty( $forum_group_ids ) ) {
			// We've found the forum_id that corresponds to the group's forum.
			$found_group_forum = true;
			$group_id          = $forum_group_ids[0];
		} else {
			$current_post = get_post( $forum_id );
			if ( $current_post->post_parent ) {
				// $post->post_parent will be the ID of the parent, not an object.
				$forum_id = $current_post->post_parent;
			} else {
				// We've reached the top of the hierarchy.
				$reached_the_top = true;
			}
		}
	}

	if ( $group_id ) {
		return $group_id;
	}

	return false;
}