Magento allows you to automatically assign customers to a customer group based on data collected from the registration page using Magento events.

<?xml version="1.0"?>
<config>
   <modules>
        
            0.1.0
        
   </modules>
   <global>
     
		<models>
            <Ayasoftware_CustomerGroup>
                Ayasoftware_CustomerGroup_Model
           </Ayasoftware_CustomerGroup>
		</models>
    </global>
   <frontend>
          <events>
		<customer_save_before>
			<observers>
				<customer_registration_page>
					<type>singleton</type>
					<class>Ayasoftware_CustomerGroup/observer</class>
					<method>setCustomerGroup</method>
				</customer_registration_page>
			</observers>
		</customer_save_before>
     </events>
    </frontend>
    
</config>

<?php



class Ayasoftware_CustomerGroup_Model_Observer
{

    
	public $customer_zone;  
	
	public  function setCustomerGroup ($observer) {
	$customer = $observer->getCustomer();
        ....
       $this->customer_zone = 5;
       $customer->setData('group_id', $this->customer_zone);
       ....
        
}