Instead of reading the SIGNUP FILE, I created a list called USERDIR and
changed our /WHOIS to read it's LIST file.  We add and remove users from
the "user directory" by subscribing and unsubscribing the users (with
QUIET) from USERDIR.  Users can even register themselves if they want.
 
Admittedly not the best mechanism in the world, but it was free, fast,
and it worked.
 
If a user doesn't want to be in the user directory (or if they want a
different address), they can just ask.