If it's more than 100, you can use gmail for domains. Very easy, I did it for my blogger's forum. It just means changing a bunch of settings at your host from your control panel, but Google really goes to great detail to map it out for you. Took me 10 minutes I think.
https://www.google.com/a/help/intl/en/admins/overview.html very clear information how to setup and use..
I set mine up through my control panel - simply input their desired name and make a random password for them. They inform them thats its set up! I use cpanel so it's very easy - most sites will have some sort of control panel.