JSON based data structure for group messaging

Question

I am trying to add group messaging functionality to the platform I'm working on. The concept of group messaging is more like a client ~ customerservice communication tool. As our backend stores data in JSON format only (no relational database whatsoever) I want to have an idea of what would be the most efficient way to store the messaging data.

Here are some details about it:

• Messaging is not necessarily real time
• Each message thread starts by a single client it will be visible to all customer service agents on the dashboard.
• We have to store information about when each customerservice agent has read the messaging thread.
• Either of customer service agents are able to answer to client at any point during the conversation that makes the conversation like a group messaging with client on one side and several agents on the other side.

Any thoughts on the best practice to form data structure for this application in JSON?