title: magento changing customer group of guest user
body: <p>I have a custom field in my onepage checkout, that is used in a <code>Model\Observer.php</code> to set the customer group using the following:</p>
  
  <pre><code>$customer = Mage::getSingleton("customer/session")-&gt;getCustomer();
  $customer-&gt;setGroupId($newGroupId)-&gt;save();
  </code></pre>
  
  <p>This all works great, and I'm doing this to change the Tax in the cart.</p>
  
  <p>However, for people checking out as a guest it throws an error:</p>
  
  <pre><code>customer email is required
  </code></pre>
  
  <p>Is there a way of setting a customer group for a guest, a second NOT LOGGED IN group for example?</p>