I went along for the first time on Saturday!
As pp says, newbies are in the corner being talked to for half an hour, explain the plan, give you your books and a form to fill in. We then sat for the group chat where they go through people's losses/gains, share some tips.
Newbies then paid and got weighed, set personal targets. We were given food diary sheets to fill in for the first four weeks which we need to take back each week so the leader can check it over and make sure we're getting it right.
They do make it clear that they expect you to stay for the group after each weigh in but from what I could see plenty came for weighing but left straight after and didn't stay for the group but I can see the benefits for staying for it.
First session took about 1hr 30m, this time i will arrive just in time to get weighed before the group session chat starts, so 45mins ish. But yes you do have to commit, you can have 6 'holidays' per year so as long as you tell your consultant that you won't make certain weeks then you won't be charged for missed sessions (you pay the missed charge when you return the following week) - one good thing though was if you can't make your exact session you can go to any other group you can find and just weigh in at least even if it's a couple of days early so you don't have to use holidays. Presume it gets logged your card that they swipe that you have attended a group on that week.
I'm happy so far but only 3 days in!