I believe this could be implemented, just not how the starter wanted it.
A player has a referal link.
They can give it to players signing up (it cannot be from same IP to prevent multis being made just for the perk, also only the first account of the IP on that world will get the benefit)
The bonus is say ... $100 ingame money.
The only way the recruiter can get the $100 is if the recruited player reaches a certain level, say ... level 10.
This ensures that
a) Innogames doesnt have to pay real cash
b) Multis will not unfairly unbalance the game
c) Players get a benefit
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To explain the Multi situation ...
Player 1, Player 2, Player 3
If 1's IP=2's IP, then no perk.
If 1 recruits 2, then any IP thats the same as 2's can't recieve benefits.
If 1 recruits 2, then 3 can't recieve any perks from 2 or any account they may have.