Societies are a way of life threw out all of Sensō. Members in each group have a common ground for meeting and fellowship. Even a foreigner is a friend if they are in your socity.
Society meetings are often seen as sacred and outside any other political or social differences. When members meet the only thing that matters is what the society is meeting for, and all other issues and subjects are left outside the threshold.
Though you can be in any number of Society's you wish to be in it is however a dishonor to be in any of them under false reasons. Each Society has their own rules on how to join.
Below are some of the well-known Society's that span all lands. There may be others to discover.