Untested:
/scripts/adduser.inc.php
just behind:
and those 2 lines of configcode looks to me like it has to be:
$XUSER = $Loginname;
$XMAIL = $Email;
The adduser script it the script that gets called when a user hits the submit button on a registration.
If it works, let us know. If not, also let us know and I'll dig in deeper.