Do you take the money, or do your users split it between their friends and you take a cut?

The take the money one has been done (Beeminder, and others probably), but sure different flavours are welcome.

The split with friends I haven't seen. That would be pretty cool, especially if you can do it without needing to pay banks a txn fee, or perhaps a tiny one (maybe you take a 5% of profits fee from participents, or similar). Crypto is the other option (!).

The split with friends is great becuase

* People less likely to lie to friends than a SaaS. Which is good for both parties.

* Social motivation and financial motivation.

* If you lose it's like losing a game of poker with mates, not a big deal (as long as you kept the stakes reasonable).