Just a small reward for bringing new paying customers to the site.
Basically when someone joins, on the sign up screen you have a spot where they enter in how they found out about the website. (which this helps you to also see which of your marketing techniques is actually working, ie. found it through another website and which one, newspaper ad, etc)
If they choose "referred by person", a new window appears where they enter in the username of the person that referred them.
Once and IF this person actually spends 10 dollars, the person that referred them to the site gets a one-time bonus of 100 credits (or something along those lines, enough to buy some balls, credits being re-used in any case).
I can't see the downfall of this in any fashion, its a win-win. I dont think it can be cheated as a kickback doesnt happen unless the new person actually spends money so someone setting up multiple accounts to game the system doesnt benefit.
I potentially could bring many new customers in short order and will be curious about a WGT response to this suggestion.