I think 99% of the time these things fail because guys like the idea, but as soon as it's a real possibility they chicken out. Fear of strangers, the unknown, the risks...
You have the existing clubs in major cities, usually hosted in actual venues and with a long history. They only work because guys usually get their friends and partners involved in the group. So they might start off meeting one or two guys one-on-one and then move on to a group scene. Also, they have a real community involvement, a closed club, secrecy... And guys know the venue is a business and has a need to keep customers happy.
I would say the group thing mainly comes from partnerships and friendships combining, more so than guys all just deciding to be in the same place at the same time for a group tugging session. If you have a jacking buddy, chances are he has another one or two, and so on... That's how the groups are made; through introductions.
That's my take on it anyway.