It’s important to keep in mind the difference between sync and sharing/collaboration. For sync we require the same iCloud account and this is done through CloudKit. For sync you also need the same dropbox account. Importantly in both cases we are talking about sharing the entire library.
What you are after sounds more like sharing and/or collaboration where you would have say shared projects but also distinct areas of your library that are available to you only (at least this is what most people would want I’d expect). We would love to bring such possibilities one day, but unfortunately it’s not an easy one and therefore not something we expect to deliver in the short-term. Whether iCloud drive could be used in that is also still a question mark.