They don't necessarily have to have a sales team to convince merchants to put up deals. Maybe advertising can do that. Maybe they can outsource the sales people. Since all they're doing is convincing merchants it's a great idea, it doesn't matter where the call comes from as long as it's representing Groupon.

Also, they can do things like yelp does with stickers at merchants places. A lot of places have the "Find us on Yelp" stickers and I doubt Yelp went out to find & convince them to do that.

I haven't spent too much time thinking how I could make Groupon better but I don't think it's really that far out there to find a better/cheaper way of acquiring customers.