I’m interested in this too. I’m a high school philosophy teacher and I’m planning to have my students build a collaborative zettelkasten on their course material over the next year, and then write essays out of that.
I’m planning to use a shared Google Drive folder for this purpose—haven’t tested but I don’t see why it wouldn’t work.
As for differentiating people’s contributions, one idea I had was to use tags. Joe’s work for week 3 is marked #joe #week3, eg. Folders would work, too.