Think about retail for a moment. The amount of money a retailer takes in is revenue. The fact that he needs to pay a wholesaler for the items sold doesn't change that fact.
If the money is being paid to Groupon, it's revenue.
In transactional / middle-men businesses, it's more standard to only report your percent take as "revenue", rather than including pass-through dollars (e.g. PayPal only reports their fees as revenue, not all dollars that flow through PayPal). But I suppose it's unclear if Groupon is that kind of business; arguably they could be more like a retailer, where it is standard to include all dollars flowing through as revenue.
Think about retail for a moment. The amount of money a retailer takes in is revenue. The fact that he needs to pay a wholesaler for the items sold doesn't change that fact.
If the money is being paid to Groupon, it's revenue.