Sounds to me more like Groupon is going down already, at least it's being talked down. I could be wrong but I'm wouldn't buy any shares even if I could.
They probably don't mean for that to be their business model, but it is what it is.
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!
Other companies have already gone public that filed around the same time as them (although raising less $) - which may hint that institutional investors aren't buying their story (the SEC making them disclose their actual losses instead of a BS metric certainly isn't going to help either).
So, okay, maybe they're going to become profitable (in real life) at some time soon in the future and we'll all feel a bit silly. But if that's the case, then why would they be using investment capital to pay off early-stage investors now— when they're just on the brink of profitability, starting an IPO, their value sure to skyrocket? What employee decides to cash out under these circumstances?