
<a href="users_Registration.php<?php echo ((isset($_SERVER["QUERY_STRING"])?"?".htmlentities($_SERVER["QUERY_STRING"]):""));?>">click here to registeer</a>

Thank you! For anyone who might read this thread later: I had to add the php code to the text link on the LogIn page as Jason described, then I also had to edit the "insert record" Server Behavior on the Registration page and check the box "Pass original querystring". I'm not sure if that box is normally checked by default or not, I may have unchecked it when I first ran the wizard.
More details coming soon....