It's the vagueness of the original statement that is the problem. If you had said 'Groupon is guaranteed to get customers to enter your building', then that would be one thing. But you just said it was 'guaranteed marketing', which probably means something different than you intended. It sounds like you support them.

Personally, I think they have a good service, provided you know what you're doing, know how you're going to keep these new customers, etc etc. If you don't, it's a disaster waiting to happen.