I also love the way they are marketing the concept. They now let you create your own meals and invite your friends. Pretty similar to meetup.com but instead focused around just meeting people over dinner. Currently the way they make sales is simply word of mouth and press. A customer has to learn about the site (either through press or friends), go to the site, and then sign up.
Now, in the new model they are trying out, they will be getting direct sales from people wanting to throw their own dinner parties. The value proposition for you to do this is pretty high, grubwith.us takes care of splitting the check and figuring out the best menu possible.