gistfile1.aw

<?php
class MyCompany_MyModule_Model_Type_Onepage extends Mage_Checkout_Model_Type_Onepage
{
    /**
     * Save billing address information to quote
     * This method is called by One Page Checkout JS (AJAX) while saving the billing information.
     *
     * @param   array $data
     * @param   int $customerAddressId
     * @return  Mage_Checkout_Model_Type_Onepage
     */
    public function saveBilling($data, $customerAddressId)
    {
        $group = Mage::app()->getRequest()->getParam('group');
        Mage::getSingleton('checkout/session')->setGroup($group);
        return parent::saveBilling($data, $customerAddressId);
    }
}