Just make a route on your web server, making all the files available with some long, impossible to guess, unique ID that can be shared. Like https://files.<your domain>/<id here>.
If they want to collaborate, they can just post the changed file, using the auth key you generated for them set in some header field, to https://files.<your domain>/<id here>, which could automatically increment revision numbers. Then you could access specific revisions with .../<id here>/rev/<revision>.
So much easier than installing an app! You could literally just use curl as the interface! (I kid)
Hah, yeah. I do have a one line CLI script to upload a file to S3, get a shareable link, and send it to me on ntfy.sh. And my family all have ntfy.sh so that honestly is viable for some things. But still, not really all that workable for many things. And only I have this power in my family