Separate Volume per user makes sense... but to build an application where users can collaborate, I would need some way of fanning out writes to other users' databases. Any thoughts on how to do that in the context of Graft?

If you're doing volume per user, but also want to do cross-user collab you might want to change the model slightly. Rather than one volume per user, consider one volume per "access unit". For example a document or group could be a volume.

As an example, let's say your building something like Google Sheets on top of Graft. Each document would be an independent Volume. This matches how Sharing works in Google Sheets, as each user added to the Volume could either be granted read or write permissions to the entire sheet.