FYI, I've set up a Facebook group for my friends, we're doing a $5/week challenge. Everyone doing the challenge chips in $5 week - this week's pot is worth $40. Provides a nice incentive, and with only a dozen or so people in it, you've got a good chance of winning one of the pots.
The winning criteria each week will rotate around to keep things interesting - from a weekly weight loss, total weight loss, inches lost, BMI percentage lost, number of minutes exercised, etc.
I'm promoting using FitDay or other tracking journals (some already were using WW or something else). They do their weigh-ins using a form I created on Google Docs, and then we use the FB page for discussions and info to share. An ad-hoc solution, but it works!