grouppay_menu_hook.php

<?php

use WHMCS\View\Menu\Item as MenuItem;

add_hook('ClientAreaPrimaryNavbar', 1, function (MenuItem $primaryNavbar) {
    if (!is_null($primaryNavbar->getChild('Billing'))) {
        $primaryNavbar->getChild('Billing')
            ->addChild('Group Pay', array(
                'label' => 'Group Pay',
                'uri'   => 'grouppay.php',
                'order' => '100',
            ));
    }
});