I am working with Node/express/javascript/mongoDB.

   My application is one where users log in, POST key/value pairs to a REST api, and the saved data is to be shared with other users of their choice.
    As of now I have the server & front end communicating so users can POST data to a local mongodb. My next step is new users/logins, which I will approach with Passport and a local strategy.
Where I need guidance:
    Let's say I have 20 users logged in, they all add their own list of items to the db in varying quantities, 10 entries give or take. HOW do I enable their document collections to stay private to their user profile in the overall database AND then be shared with select people to view as in a private group? (is namespacing involved here, or just dynamic page routing?)
    I would appreciate advice on if I am proceeding in the right direction and some opinions of popular approach to this kind of thing.
In a nutshell: user logs in, makes a list of things (saved to mongodb), user invites X amount of friends to view their list. User can multiple lists for different occasions as needed.