This is a technicality, but there's a difference between a Ponzi scheme and a pyramid scheme. A Ponzi scheme needs an information asymmetry: investors don't know that their money is being used to pay off previous investors. A pyramid scheme is open about the way it works and the pyramid nature, but people invest anyway.

Since Groupon's investment and revenue strategies are known to all, this would be closer to a pyramid scheme than a Ponzi scheme.