New users should get their user account associated with (old) rsvp

This code will do the trick:

/**
* Implementation of hook_cron
*
* Processing assuring that new users get their invitations associated with their account.
*/
function rsvp_cron() {
$result = db_query('update rsvp_invite r, users u set r.uid=u.uid where r.uid=0 and r.email=u.mail');
}

It would be more elegant to calculate the uid on the fly, but this works too. Hope this helps.