What are good platforms for collaborative writing with multiple senior authors with limited technical background?
If you are in a hurry, you might want to try an online TeX editor such as Overleaf.

Another solution if you are using TeX would be to work alongside a version control system, e.g. git. Keep in mind that it takes some time to get used to it, though.

I do not know about non-TeX-solutions but i feel that a plain-text format has a couple of advantages for this kind of application.

I am looking forward to read other responses to learn about other options.