https://www.collaboraoffice.com/ is LibreOffice in the cloud.
Onlyoffice can also be self hosted as a cloud office solution (can't remember if that one is 100% FOSS).
All about open source! Feel free to ask questions, and share news, and interesting stuff!
Community icon from opensource.org, but we are not affiliated with them.
https://www.collaboraoffice.com/ is LibreOffice in the cloud.
Onlyoffice can also be self hosted as a cloud office solution (can't remember if that one is 100% FOSS).
Onlyoffice
AGPL-3.0-only license, which from my limited knowledge means it's FOSS. Collabora looks great! Though seems extreme for our use case.
Joplin has a sharing feature that allows multiple user editing of an entire notebook.
The downside is that both individuals need to have Joplin installed and either have Joplin cloud accounts or a self-hosted installation of Joplin server, which in either case may be more effort than is worth it for you.
Etherpad is a relatively lightweight option: https://etherpad.org
And you can find some server here : https://github.com/ether/etherpad-lite/wiki/Sites-That-Run-Etherpad
We are using Hedgedoc a lot, and we love it!
Abiword and onlyoffice are good options too
I like CryptPad a lot.
Collabora Office is nice too but I prefer CryptPad for my purposes