well there is a mass email invitation hack... where in members can send invitations to their friends by inputing their email id and password.

Try this...
If you have a gaming site, have a gaming clan/guild!

When your members are out on gaming servers, they see your websites name.
I ask my member to bind our website URL and say it whenever they win.

Whenever I do really good in css, at the end of the round i say "and while your dead, go join v-gaming.net" haah