It’s mostly a server-side configuration: you’d want to define a channel for each user (probably named after their user ID for simplicity). Then you could have a sync function that maps a document to a channel based on its docID, so for example document “userInfo-jens” maps to channel “jens”, so I and only I have access to it.
Then on the client side you simply create/update the document with that ID and store whatever you want onto it.