gistfile1.txt

    protected function _prepareCollection()
    {
        $collection = Mage::getResourceModel($this->_getCollectionClass());
        $collection->getSelect()->join(array('customer_info' => Mage::getSingleton('core/resource')->getTableName('customer_entity')), 'main_table.customer_id = customer_info.entity_id', array('customer_info.group_id'))
            ->join(array('customer_group' => Mage::getSingleton('core/resource')->getTableName('customer_group')), 'customer_info.group_id = customer_group.customer_group_id', array('customer_group.customer_group_code AS group_name'));
        $this->setCollection($collection);
        return parent::_prepareCollection();

        }