First of all welcome!
I have thought about groupon but never really pulled the trigger on it. I don`t focus my business on trying to hoard as many clients as possible. And people who are looking for groupons are typically the ones looking for the cheapest price, while simultaneously expecting top price work. Ive had competitors use them, and have lost business to them (due to my unwillingness to budge on my price because "so and so has a groupon") but the fact of the matter is those are exactly the clients I DONT want anyway.
It really depends on what your focus is going to be. Do you want to run a volume based business where youre kicking out 3 4 5 cars a day (this entirely depends on how many of you there are, assuming just you)? Groupon may be a good start to get your feet wet. If youre going out there trying to provide top quality work and truly detail cars, id pass on it.