From the sound of it, there is no direct link between this GUID you have and a User. You may need to build an association table. You would populate this table when the user registers with their new userId and the GUID you generate. Then, when they click the link in their email, you would query that association table to find which UserId is assocaiated to the GUID in the querystring. From there, you can update the user table.
Another option would be to perhaps pass them a different code that is a mix of their UserId, a timestamp and a guid. Then you would simply know where in the string their userId is placed. That may be pretty easily exploited though.