Help with send to and reply to functionality

I am trying to create a system where a user can create a document and send to one or more other users, and allow them to reply to it. I’m not sure how to most efficiently model this in regards to users, inbox, and mongo. So, a user creates a document, send to one or more users, the document shows up in the user’s inbox as new, and they can reply to it. For instance, should I just create an array for the recipients and then parse that array and send to each one? But, what about replying? I have never created this type of functionality and I want to set it up correctly from the start instead of doing it how I think would work and the whole system has to be replaced soon after implementing it.