Real Time Collaboration

I'm surprised this isn't already a thing.

You mean like what Google Docs, CryptPad, ... do for text or spreadsheets? Kind of cool I guess, but likely very hard. Currently there's only viewing & playback in the browser (and mobile apps), but no editing at all - I'd assume that would be the minimum requirement?

Can you give some examples for how you would actually use it?

A workaround you can have right now: Open MScore on one PC, then the others use desktop sharing software (TigerVNC, TeamViewer, ...) to remotely log in. That way, they can see what happens on the screen, and take turns controlling the mouse and keyboard.
Or, if you need to work on different parts at the same time: Every collaborator has their own version of the score in a shared folder (a Network-Attached Storage, Dropbox, ...), and also opens all others in tabs or side-by-side, read-only. ("Auto-reload when file changes" would be helpful). You look at / listen to what any other have written, and if you like it, merge them manually into your version.
Both may be not super convenient, but you could try them right now. They're also less likely to degenerate into total chaos. (A score has a more complicated structure than a linear text file, which makes it easier to step on each other's toes, I assume.)

flat.io can do real time collab, the software is more basic for beginners too. not much you can do without premium anymore, which is a huge shame