I've long relied on skating and walking for weight control, never any fad diets or support programs. The closest I've come to an organized group was The Fat Man's Club when I worked at Wang Labs in the mid-80s. Prospective members showed up on the first workday after New Year, weighed in, and put $25 in the pot. Successful members showed up 15 pounds lighter on April 1st and split the pot. No other meetings, no advice, no support, just the prospect of a payoff. (I made the grade and collected $45.)
Now a current co-worker has put together something similar, but with more checkpoints. At each weekly weigh-in, you pay $1/pound of difference to everyone who lost more weight than you did and receive $1/pound if you out-dieted them. Moving with the group average is a financial wash. You win money by pulling ahead and lose it by falling behind, so there's your cash incentive.
I might be at a disadvantage from having the lowest starting weight in the group. (It's not that my co-workers are so fat, it's that they're taller than I am.) I'll let you know how this develops.
Today: 13 miles. This year: 202.3 miles.