It's simple. When someone you refer signs up, be sure they enter your username or domain name in the "Friend or website who referred you" field during the last step of their account setup process. There are no limits to how many referrals you make, or how many credits you earn.
*Please note, Referral credits cannot be credited retroactively. Your domain or username must be indicated by the person you refer at the time the account is set up. Both your account and the referred account must follow the Referral Program Terms and Conditions to receive credit. If you have any questions please email us at firstname.lastname@example.org