Well i think that you can't have a chat solution with 100 members or even more without consume your server resources.
100 concurrent members generate a huge amount of traffic, there is nothing to do to avoid this.
If you want to not overload your server maybe you should try an alternative approach, based on hosted chat or shoutbox that runs on third party and remote servers.
By the way i meant public chat as the public window with chatrooms, against to the private chatboxes that can be opened one-to-one between users.