bbp_set_current_user_default_role()

function bbp_set_current_user_default_role() {

	/** Sanity ****************************************************************/

	// Bail if deactivating Forums
	if ( bbp_is_deactivation() )
		return;

	// Catch all, to prevent premature user initialization
	if ( ! did_action( 'set_current_user' ) )
		return;

	// Bail if not logged in or already a member of this site
	if ( ! is_user_logged_in() )
		return;

	// Get the current user ID
	$user_id = bbp_get_current_user_id();

	// Bail if user already has a forums role
	if ( bbp_get_user_role( $user_id ) )
		return;

	// Bail if user is marked as spam or is deleted
	if ( bbp_is_user_inactive( $user_id ) )
		return;

	/** Ready *****************************************************************/

	// Load up Forums once
	$bbp         = bbpress();

	// Get whether or not to add a role to the user account
	$add_to_site = bbp_allow_global_access();

	// Get the current user's WordPress role. Set to empty string if none found.
	$user_role   = bbp_get_user_blog_role( $user_id );

	// Get the role map
	$role_map    = bbp_get_user_role_map();

	/** Forum Role ************************************************************/

	// Use a mapped role
	if ( isset( $role_map[$user_role] ) ) {
		$new_role = $role_map[$user_role];

	// Use the default role
	} else {
		$new_role = bbp_get_default_role();
	}

	/** Add or Map ************************************************************/

	// Add the user to the site
	if ( true === $add_to_site ) {

		// Make sure Forums roles are added
		bbp_add_forums_roles();

		$bbp->current_user->add_role( $new_role );

	// Don't add the user, but still give them the correct caps dynamically
	} else {		
		$bbp->current_user->caps[$new_role] = true;
		$bbp->current_user->get_role_caps();
	}
}