Groupon is not a Ponzi scheme. In the process of raising money from late stage investors, Groupon has to say what the funds will be used for, which includes paying off earlier investors.
Groupon is making a fist ton of cash. If they stopped spending money on hiring and marketing, they'd be wildly profitable. Investors have access to all this information.
Also, Groupon has a clear formula for growing and how long it takes that new business to become profitable. That is why they can raise so much investment.
If you can come up with a convincing argument why Groupon is a Ponzi scheme, then call the law enforcement. You'll get on the news for sure!
They probably don't mean for that to be their business model, but it is what it is.