There is a really cool feature of DW that allows you to copy a recordset from one page to another. You can select the recordset on your detail page within the server behaviors area and copy it with Ctrl+C or Cmd+C if you are on a pc or mac. You can then open up the new page with the form on it and paste in this recordset into the server behaviors area.
This will duplicate the server behavior, you can then just link to this page with the same identifier that you use on the details page like this:
<a href="DetailEmail.php?UserID=<?php echo (isset($_GET['UserID']))?$_GET['UserID']:'' ?>">link to email page</a>