Can a group of people have the same IP address?

I moderate a forum and a fellow moderator and I have became suspicious of a particular poster and checked her IP address and seen it said users posting under this IP and there was a list of other posters who we have trusted and believed to be legit..Does this mean that the poster we were checking is really all these people? I find it really hard to believe that some of these posters could be her but never know. I don't know a lot about how IP addresses work but I thought every individual computer had there own specific IP, How can a several people on one forum share one?