Public AIM and/ or iChat Chatroom

I would like to make a public chatroom, like one that people can go to file< go to chat room< and enter the chatroom name. (People on a mac of course) I don't know how people on PC's could join, but that doesn't matter. Some kids at my school made one for kids with accounts. I joined it a day late, but none the less, it existed. I cannot figure out how to make one for the life of me! Please help.