There used to be a link in the User Control Panel, which was customised for each user. It allowed you to send a link to GTP to someone, and if they registered it would automatically add one to your referrals total.
AltF8 has the most referrals because he used to run a sub-site here called Gran Turismo Speed, and I think hed drove about 200 people here through that. It isn't running any more though.