Mongodb Design of Chat System

  mongodb, question

I want to make a chat system with mongodb and nodejs. He has the function of adding friends and grouping friends. I don’t know how to design such mongodb better.

 {grounp:' Grouping 1',list:[ User ID1, User ID2]},
 {grounp:' Grouping 2',list:[ User ID3, User ID4]}

People clearly asked mongo, but everyone upstairs answered redis. redis is useful for redis. It works well on one-dimensional linear data, but mongo is still needed for data similar to relational data.