A ponzi scheme is, by definition, zero sum. And it is, by definition, opaque. New investors pay-out old investors. This was a buy-out of early shareholders. If you expand buy-outs to be considered a "ponzi scheme" than so is anything where somebody buys an asset from somebody else aided by a broker.
When the payout is a dividend, both the new and old investors feel that their principle is remains and they could cash-out at will.
What you have with Groupon was certainly bad PR to people like yourself and thousands of others who are tuned-in to this sort of news. But it most certainly was nothing like a Ponzi scheme.
First, it's not zero-sum. The "machine" of Groupon produces $1.x dollars in value for every $1 of capital.
Second, the new investors knew when they invested that many of the shares they were buying were being held by existing shareholders and did not come from the company's pool. They knew that to acquire the share of the company they wanted to hold, they'd have to buy-out these earlier investors and that it would of course be at a premium to their initial investment.
Finally, the early investors knew that they were being bought-out, that they were selling their shares to a 3rd party.