Well for that I'd suggest to enable the forums "referral" system (it's there on MyBB dunno about SMF). It probably should be available with a plugin. With that you can track how many users someone has referred the forum to. Or perhaps at registration you can make a field asking users who referred them. Good Luck, hope YGG is able to regain it's glory (surely will).
PS I already have 3 of my maps on the server, does that count? lol
#GoYGG