Bonus for bringing new players?

[Spark910]

They could do rewards based on groups of referals.

So, 1-3, 4-5, 6-7, 7-9, 10-15, 16-25 or whatever, then it's less abuseable as there is not a reward for each referal as such, and they can set a maximum, say 100 in a year, or space the last groups out far, like 100, 200, 300 etc...