Here's a recent state of things... It's a bit of a moving target, but they get a big chunk up front.

http://venturebeat.com/2012/05/07/groupon-tightens-its-payme...

Another thing to realize is that they get the money REGARDLESS OF WHETHER YOU REDEEM IT. EVER. Depending on the type of Groupon, upwards to 20% of them are never redeemed (this is a dirty secret of gift cards, too).