K/S Circle is the first online K/S forum. It was begun by a group of fans who been members of the long-running apa called K/S & K.S. (Kindred Spirits).
One fan remembers:
I had been a member of the K/S & Kindred Spirits Amateur Press Association (APA), which had been running since the mid-1980s. It was a private correspondence group that put out a print publication every other month or so, which was (snail) mailed only to the members. Around 1995 and 1996, a few APA members acquired accounts and began emailing one another, individually. At some point it occurred to me that it might be fun for all of us to be able to participate in one another’s conversations, as we had in the APA. We kicked around the idea, discussing the pros and cons. The first “pro” was that we wouldn’t have to wait two months to hear back from the others. However, some people were quite leery about what such a group would mean to an individual’s privacy. We decided that the K/S Circle would be a private group, similar to the APA. Any current member could propose (and vouch for) a new person for membership. All members had to agree to the ground rules, which included using our real names and not allowing anyone outside the group access to the group’s postings. The K/S Circle has been in continuous existence since 1996. At first, five or six friends e-mailed the whole group by cc-ing the others. Then someone found a host site that we used for a while. Membership increased to around twenty-six, losing and picking up various people along the way. Eventually, in August of 1998, the K/S Circle moved over to Yahoo Groups
where it remains today. As open-membership K/S groups came into existence, activity on the K/S Circle slowed down, but it hasn't stopped entirely." 
- ↑ from The Internet: K/S and Internet Connections