You can turn on the option to not allow duplicate e-mails, in which case they'll be given their username if they try to sign up. The main problem here is that one e-mail address could be used for 50 different accounts; I don't think you'd want to e-mail _all_ of them.
Jason Rhinelander
Gossamer Threads
jason@gossamer-threads.com
Jason Rhinelander
Gossamer Threads
jason@gossamer-threads.com