Updated to add my own referral link to this deal. If you're not a member yet, you'll get a $10 credit, and you can start referring your own friends right away. If you're already a member, click on "Give $10" on the top menu.
Sign up for an account and invite your friends to join. Your friends will get a $10 credit when they sign up, and you'll earn $25 credit the first time they travel and $200 the first time they host.