gistfile1.txt

protected function _prepareColumns()
    {
    ...
            $this->addColumn('group_name', array(
            'header' => Mage::helper('sales')->__('Customer Group'),
            'index' => 'group_name',
            'width' => '70px',
            'filter_index' => 'customer_group.customer_group_code',
        ));
    ...
    }