Most unix installations support use of .forward files (don't forget the .)
You just make a .forward file in the user's home directory with a list of names to forward the mail to and off it goes. I use this all the time for notification of groups who maintain different parts of our intranet at work.
It's real easy to write a script that edits that file so the user can set the address themselves, or you can do it if you're worried about them using it for spam.