So the last to deposit before the round ends gets the jackpot, but people before him get nothing (depends how much they need to pay for the first deposits)?
Quite a gamble.
More like quite a ponzi scam!
If they describe what they do and even call it Ponzi it isn't scam but a game

And the end is actually rather a lottery since usually no one who is on the end of such a scheme does get anything at all.