Outbound sales I'd imagine. It takes a lot of people to make calls to businesses to convince them to sign up for Groupon, how to structure their deals etc.

Wasn't that a scene in Silicon Valley? Their competitor had like 3 developers and 100 salesmen.

That sounds about right though. The developers are really just building a CMS that should be able to in theory handle an infinite amount of cities, while securing a new sale for one city every day could be a full time job (and probably done best by someone actually in that city).