They might be easily forced, but I don't think there has to be a special secret society in place or a rota.. more like one person gets a key, and it grows from there; 'hey mate, how did you get in there?', 'I got a key from eBay, there's loads on there and they only cost £xx. Much better than being cramped in there isn't it?'.
When I commuted for a long time on the same train (the 0844, hence my username) I saw the same people day in, day out - all sitting usually in the same seat/area. In our case, first class at the rear end of an 8-car 365, as it was declassified. One Christmas, one person in the group came along with cake, christmas crackers and a bottle of champagne (yes, in the morning) and we had a mini party - which even included an RPI that had to refuse the offer of a drink!
So, it wouldn't be impossible for there to be the same people doing it on at least one individual service almost every day. Don't be surprised to learn that while a lot of commuters never talk to each other, it doesn't take much for them to do so (say, one day when there's disruption and people begin to chat/unite) and then because they see each other regularly after, do form some sort of unofficial 'pact'.
