You should be override some of files for add your custom variable in mail.
First copy app\code\core\Mage\Sales\Model\Order.php
to app\code\local\Mage\Sales\Model\Order.php
Create function for your requirement.
If your order data return value for customer group as $order['customer_group']
then you have to declare function in Order.php
file of local.
public function getCustomerGroup(){
//find customer group from order customer and return it or load order model to get customer groupid.
return $order->getCustomerGroupId();
}
You have to set value in System -> Transactional email -> New order template.
You have to set below statement in your email template.
{{var order.getCustomerGroup()}}
Now email template will display Customer Group value.
Set this email template from System -> Configuration -> Sales -> Sales email -> Order -> New Order Confirmation Template -> Save.
Hope this will help you.