Here's how it works: you sign up and pay a certain amount of $ (I was thinking $25?) and weigh yourself. You submit the weight and a photo of it to the site. We all get 28 days to lose 4% of our body weight. If memory serves it never reveals your actual weight to the group. At the end you reweigh and the people who make goal split the pot of money. We have a message thread there to post things and encourage each other.
Here's the page with more details: http://www.dietbetter.com/dietbet-4/how-it-works
I'm excited to try it again because I think it will help motivate me to make good choices as school starts again and our routine changes. I figure a contribution to a dietbet is way cheaper than buying the next size up in fall clothes :-p