You could customize the file:
/includes/modules/order_total/ot_group_pricing.php

and get the group name to display with the code in RED:
Code:
      $order->info['tax'] = $order->info['tax'] - $tax;


// bof: add group pricing name to title
    $group_query = $db->Execute("select customers_group_pricing from " . TABLE_CUSTOMERS . " where customers_id = '" . (int)$_SESSION['customer_id'] . "'");
    $group_discount = $db->Execute("select group_name, group_percentage from " . TABLE_GROUP_PRICING . "
                                      where group_id = '" . (int)$group_query->fields['customers_group_pricing'] . "'");

      $this->output[] = array('title' => $this->title . ' ' . $group_discount->fields['group_name'] . ':',
      'text' => '-' . $currencies->format($od_amount['total'], true, $order->info['currency'], $order->info['currency_value']),
      'value' => $od_amount['total']);
// eof: add group pricing name to title