Some of them formed because we had something in common. All young, all old, long cycles, had MC in the past. Others, are just groups formed so that people could be in a group and chat with others and get to know the people in their group a lot better than just from the occasional posts. Almost all are open and welcome to new members, so if you see one you like (or two) just post a message asking to join. Many have a little jpg that you can put in your signature to show you're part of the group.