My group is so small there's no option but to let people come and go - although maybe that's just because there are far fewer men than women. A couple of times I've been the only one there - but at least my LLC is happy to run sessions with just me, and because I'm getting so close to target we've been leaving the books aside and talking about the kind of things I'll be doing once I start RTM.
I've had details of the new RTM format. Instead of 12 weeks, it's 8 stages. You can stay on a stage longer than one week if you want. The foods that get introduced at each stage are phased in based mainly on their Glycaemic Index, so that the carbs start to build up very slowly, with slow-release ones coming before quick-release ones.