This is obviously a "loss leader" just to get you in the door, where they will try to "sell you" more dentistry. If you are actually getting ZOOM (ask to see the label on the materials used), and the staff does it properly, then you found a bargain and take it!
ZOOM Teeth Whitening for $99 is definitely a good deal. The lowest you can get it for without a Groupon deal is around $150. So take it. But please research the dentist, look up his/her reviews and do your due diligence before stepping into the office. I am sure you realize that once you step into the office, they will try and sell you additional services - nothing wrong, that's why it is called a promotion!
I use Groupon to attract new patients to my office and treat every patient exactly the same as ones who pay full price. I do not "sell" patients on more treatment than what is absolutely necessary. We have done hundreds of zooms using groupon and most if not all patients are extremely happy with the way our office treats them. Not all offices are the same but I would think you should be fine.
The question with any discounted product is "What are you getting for your money?"
Check out the dentist office very carefully. If they have a good reputation, do quality work, and you can find no bad review, then go for it. If they are smart, they will offer a promotion and always give you more than you expected in service and quality so you will come back.