I run a company focused on real-time collaborative software: https://convergence.io

As a means to improve the status quo, we're looking for some really great (and really awful) examples of multi-user co-editing software. In our opinion, there are a whole lot of poor examples out there: totally broken undo/redo, not knowing what other people are doing, etc etc.

Some common examples to get you started: Google Docs, Figma, Invision Studio