PHP / MySQL Sort Code Multi-Table

[roboticsguy1988]

Alright I have a odd question... but first let me layout the table scene.

 

I have a database which has a user table in that user table is information such as UID (which is a unique user ID), First Name, Last Name, etc.  Also in that database are "groups" of people.  Each group is a table in the database in that table the UID is listed to match the user table.

 

What i want to do is sort one of those group tables by the last name but the last name is in the user table and not in the group table. and before anyone mentions it no they do not want the a last name column added to any of the group tables.  Essentially this database uses keys so the only duplicate data is keys which are use to match with other tables and pull the other information from the other tables.

 

Also, i would like to run the sorts from a php script.

 

Hopefully i explained all of that well enough, if you have any questions please let me know and thanks in advance for the help.