title: Add Customer Group filter to Sales Report in Admin
body: <p>I have a site that is currently running version 1.3.  Unfortunately due to changes made in the code I am unable to upgrade the site at this time to a newer version.  I am trying to add a drop down filter to the Reports -> Sales -> Sales Reports for Customer Group.  I am able to add the front end filters by creating a new grid.phtml file.  However, I am not sure how to include the dynamically chosen customer group and filter it in the collection.  </p>
  
  <p>Does anyone have a suggestion on which collection should be extended to achieve this (I was trying to extend 'Mage_Reports_Model_Mysql4_Order_Collection') or how to call the dynamically chosen variable? Would it be better to create an entirely new custom report? </p>