[zappsan]

You need to turn on the referral system first.

When someone signs up, there is a field where they can put the person's username who told them about the forums.

People can also use referral links when they invite people so the name will be filled in automatically at registration.

People can find out their referral link here:
yourforums.com/faq.php?faq=vb_board_usage#faq_vb_referrals_explai n

It's yourforums.com/?referrerid=userid where userid is the ID of the referrer.