Bulletin Board System / Bulletin Board Service.
Old days, modems hooked up to phone lines instead of permanent.
People set up their own computers to receive phone calls and the caller would see lists of option on their computer. They could send messages in groups or individually, send files to each other and play games with each other.
Usually it was just one phone line in, so it would be one at a time.
But they were usually very local, so it was all local area computer geeks from around town.
After a while they’d go and hang out together in real life and stuff, make friends / enemies etc.