WordPress at a glance
function is not described

wp_admin_bar_add_secondary_groups() WP 3.3.0

Add secondary menus.

No Hooks.

Return

Nothing (null).

Usage

wp_admin_bar_add_secondary_groups( $wp_admin_bar );
$wp_admin_bar(WP_Admin_Bar) (required)

Code of wp admin bar add secondary groups: wp-includes/admin-bar.php VER 5.0.3

<?php
function wp_admin_bar_add_secondary_groups( $wp_admin_bar ) {
	$wp_admin_bar->add_group( array(
		'id'     => 'top-secondary',
		'meta'   => array(
			'class' => 'ab-top-secondary',
		),
	) );

	$wp_admin_bar->add_group( array(
		'parent' => 'wp-logo',
		'id'     => 'wp-logo-external',
		'meta'   => array(
			'class' => 'ab-sub-secondary',
		),
	) );
}