I associate features like this with etherpad[0] which I was in the habit of using for years for collaborative projects.
[0]: https://etherpad.org
Or Dropbox Paper which has origins in Etherpad.
Huh, Etherpad was acquired by Google in 2009 and a fork of Etherpad, Hackpad, was acquired by Dropbox in 2014 [1]. Both projects got folded however: Etherpad into Google Wave, and Hackpad into Dropbox Paper.
Etherpad lite is still alive: https://github.com/ether/etherpad-lite