First and foremost, check your webhost control pannel. You should have a mailing list option in there (my host uses CPanel X, Mail > Mailing Lists). I'd test it out with a your own private accounts, or get a couple of friends on ventrilo/icq/etc. and test before going live to the 200+ users.
Second, check and see if you have any auto-install scripts in your control panel (CPanel X, Fantastico > Mailing Lists).
If those don't work, however, give this one a whirl.
PHPList