At first I feel like it was the only one, or at least the only prominent one, then at various times various groups sprang off of it. once it became a little easier to create groups on Yahoo groups there were all sorts of Cosplay groups, like I was in one that was specifically just for Florida cosplayers because at that point in time we were like our own little isolated island in the cosplay sea.
People also created alternate groups for various reasons, I think at one point some people got annoyed that the things they wanted to talk about were being considered “off topic” so they made their own group where you could talk about cosplay AND whatever else, etc.
As a whole though COSP was very much a reflection of cosplay at that time, it felt like a very small intimate group at first and even continuing into the early 2000s. People would make posts like “I was dressed as this at this con, did anyone get my picture?” people would ask for advice about something they were trying to do or even offer advice at some points if it was something they did that they wanted to advise others about, but usually those sorts of things were hosted on people’s personal webpages and they would just mention that they had a tutorial for this or that and link it, there was definitely a lot less tutorial sharing in the sense that it is today. Really mostly what it was, was resource sharing. like if you found a good place for wigs, or a place that would ship a particular material that wasn’t terribly expensive, you might post on there and tell people about it.
There was also a lot of organizing cosplay groups, like if you had a group you wanted to do, you might post on there to gauge interest for either a skit or just a bunch of people cosplaying from the same series just at the con. Like “I’m planning on cosplaying from slayers at otakon, is anyone else going who also is?” etc
People would post links to photo albums of all of the photos that they had taken and there was a small handful of photographers who would also post on there Kevin Lillard of fansview being probably the most prominent and active.