I occasionally see requests for collaboration functionality in Agenda.
I use Notion, Quip and GDrive for collaboration with different clients. Based on that experience I’m pretty certain I would not want to use Agenda for collaboration!
Agenda is my personal space, with my structure, my quirks. I don’t have to thing about making it work for anyone else or be constrained by other people’s expectations - which is a pain with the other services, though they have other strengths.
That said, I do sometimes start writing in Agenda and then want to get feedback on it from other people. Being able to share a single note for that purpose could be useful. I’d want comments and suggestions. I wouldn’t want people to be able edit the text without me being able to roll back changes. And that takes me back to the fact that for me, Agenda is personal: I’d like feedback on a note, but it’s still my note, I have the final say on what goes in it!