I think I found another way to explain it. Every new user who registers, will automaticly have in his profile a tab :your referral ID is
www.mysite.com/?referrerid=x
If I make a competition
he can than choose to use that referrid to invite friends. In his tab he will also see how many friends registered with his referrer link
and of cause the admin can also check this wich means I can track this also in the backend. I think this might be aspecially good for new siteowners like me:laugh: who want to giver their new user an incentive or a price for most invited new users