I've struggled with this topic for a long time since I edit scores with some friends at the same time, and came with some workarounds, but none of them seems that efficient for the workflow.
I mainly use MuseScore as it is very useful (and I'm eager to see the changes for MuseScore 4 when they release it). But real-time collaboration doesn't seem to be planned anytime soon.
As a workaround, I found useful to use Box Drive for saving any change and sharing screen on Discord, but the issue is that the other people cannot do anything but to speak until I stop screen sharing, close MuseScore, and then someone else opens the file and shares screen. Having only one editor is not useful for efficiency.
So, as we write for real performers, the main focus is notation, and MIDI/Playback lays on second place (at least yet). And, as I've worked with LilyPond for quite a while, I thought it could be easier to work with LilyPond as for real-time collaboration, since it's just text.
So I tried what I thought could be useful for that: VSCode, "VSLilyPond" Extension by lhl2617 (this guy is awesome, thank you for answering my previous questions), and "Live Share" Extension by Microsoft.
The big problem with this is that Live Share doesn't support binary files, and that means PDF and some other formats. That does not only affect LilyPond users but also LaTeX users.
Then I thought that a workaround to this might be a PDF reader that reloads itself when any change happens.
I use a Mac (with MacOS High Sierra 10.13.6, but probably I'll be upgrading to Mojave when MuseScore 4 is released, which is the last MacOS version that runs LilyPond), so looking for a useful resource, "Skim" fits the best for that. But when working with files in Box Drive, it always crashes when someone else saves the file (it only reloads correctly once or none, and with too much latency). Should I instead use Preview.app and change some things with XCode to disable the sidebar to see if it works?
I'm not sure about Windows nor Linux, but I've read that for Windows "Sumatra PDF" works nicely (I tried it with Wine on MacOS and works well), but I don't know if anything like what happened on MacOS with Skim would happen as well. And the only information I have about Linux is that Evince works for this, but I wonder how it differs with files in Box Drive (is there Box Drive for Linux distros, though? If not, that could be another issue if someone uses Linux. Maybe use another service?).
If someone knows anything that might work, even if it's not too straightforward, or even complicated, I would appreciate any ideas.