I think you're all over thinking this a little, all that's required is a referral code.
If you've ever bought something off the internet, 9 times out of 10 it'll have an option to enter a referral code at checkout. Entering this code gives yourself and the person you recommended a reward of some sort, 5% off both of your next purchases for example, thus giving both parties an incentive to give out/use a code.
If we translate this into a referral system on opticraft all it requires is a new member to enter an existing members referral code in the registration process. A reward for using a code could be $1000 in-game cash for both parties.
No, that idea is open to abuse.
I can easily imagine smart guests going "Hey, I recommend you, you recommend me? Cash for both of us!"
What you just said is the whole point of this system. -.-