Strategy first, tactics second.
Unless you've already got a strategy and processes in play for retaining and developing new business with/through existing customers, you're wasting your time (and money) with loss leaders. What's more is that using a service like Groupon risks attracting the kind of customers you don't want and don't need... and therefore don't fit the strategy. Plus, if you go looking, you'll find stories of small business owners who had no idea what they were getting themselves into when they signed with Groupon and were pushed to the brink of bankruptcy.
So my advice (for most people) would be to stay the hell away from Groupon. Do something more strategic and sustainable with your marketing.
Apologies for sounding so categorical and stern. Think of it as tough love. That's my intent.
This, and if you REALLY want to go down this road...don't use Groupon, use LivingSocial.