I have observed a similar pattern at American folk festivals, especially the Northwest Folklife Festival in Seattle. There are easily half a dozen different communities with only very light overlapping members. The contra dancers, the old-time fiddle folks and the group-singing enthusiasts (among others) seem to have their customary places and rituals.

This is not to say that any of these groups are closed to "outsiders" unless the outsiders are interfering with the group (such as an off-the-beat spoons player). They're usually happy to welcome a newcomer who has a new song or tune to share, if it's within the understood limits of the group's interest -- for an extreme example, if somebody moved into a fiddle session and knocked off a Bach sonata, they might get some funny looks.