The Libre blockchain is the first blockchain in the world (we think) that was built with the idea of on-chain referrals. The way it works is simple:
Every account (example: @bob) can refer multiple other accounts (example: @alice and @susan). This parent-child relationship is stored in a table on the actual blockchain. Unlike humans, each account has a single parent however, or no parent at all. Going back up a level, @alice might be referred to by @bob, but @bob was referred to by @mike.
Now when @alice joins the system, and spins the wheel -- @alice will make a random rewards in LIBRE tokens. Some percentage of that (10% in the initial spec, but modifiable by the DAO) will go to @bob and an even smaller proportion to @mike. This is an incentive program at work!