Ko is a society organized by people having the same faith. Note, however, the term 'ko' is used in a wide variety of organizations such as Mujin-ko (beneficial association).
The term ko originally represents 'kogi (lecture)' or 'ko-doku (reading),' and referred to a group of monks living in the Heian period who read and studied Buddhist scriptures. It later came to mean 'ko-kai,' a Buddhist event focused on the reading of Buddhist scriptures, and the use of the term spread to a wide variety of Buddhist rituals (such as Hoon-ko (a memorial service for Shinran)).
While this concept of ko spread to the public in the Middle Ages, many types of faith groups used the term ko in their names. Among ko as faith groups, some were naturally formed in local communities and some were introduced from outside. The first kind of ko was operated by ujiko (shrine parishioners) who believed in regional deities such as ujigami (a guardian god or spirit of a particular place in the Shinto religion) and ubusunagami (guardian deity of one's birthplace) in order to maintain their shrine. Some ko of high-class shrines have members even outside the village it is located in.
Ko is also called 'kosha (a meeting for the purpose of spiritual guidance or for conducting a religious ceremony, or an organization for holding such meetings),' and members of ko are called 'koin.'
For ko operation, officers such as komoto (host of the ko), fuku-komoto (vice komoto), and sewanin (manager) are appointed. They are usually selected from ko members and are appointed by the shrine or temple that the ko has faith in.
Ko that were introduced from outside were originally related to mountain worship. Ascetic Buddhist monks from Tate-yama Mountain traveled across the nation to encourage people to climb sacred mountains, and built sanpai ko (ko for temple or shrine visits) at various locations. Following this, a large number of ko for shrine and temple visits were established in various parts of Japan. These sanpai ko held 'so mairi' in which all ko members visited their temple or shrine, but in many cases, 'daisan ko (ko by representative)' was held in which a few people were selected as representatives to visit their temple or shrine on behalf of their ko.
Use of the concept of ko in beneficial organizations (such as Tanomoshi-ko and Mujin-ko) has been derived from the concept of daisan ko. In these organizations, they collect money from members but instead of visiting a temple or shrine, they buy lottery tickets or bid in auctions, and money won in lotteries or objects obtained through auction are given to the members.