Is it possible to share iCloud Notes with non-Apple device users?
The web app for iCloud notes is quite accessible cross platform. I don’t think you can invite a random non-iCloud account, but have you tried that?
I do know photo sharing works for people on Linux / windows (pc and phone) and android using web version of the iCloud photos, so notes or files might be next to ship.
Worst case, get them to create an AppleID / iCloud and you’ll be able to collaborate with them going forward even if Apple doesn’t open up notes sharing more widely outside of registered accounts.