One other thing. Looking at the last weigh in thread about 25-30 people posted. I am not an admin and can’t see how many 30 day groups we have (and don’t want to do data mining on the sign-up thread), but it can’t be that many. I think the deposit would discourage people even more from participating.
As @Crispy put forth, just increase the group size to account for possible attrition. Not to mention people join gyms, buy exercise machines, books on diets etc. at a much higher cost than the deposit suggested and let it go to waste.
I am more of a rewards guy. I fixed @admin post to say “One of the solutions was to give guys an Amazon gift certificate of £5 or £10 per group that would be awarded when they finished their 30 days.”