How to map two users using Conversation Id

[nhkrishna5]

How to maintain the same conversation Id between two users for chat application using "AWS Appsync"
Ex:- If "user A" sends a message to "user B" for the first time a conversation Id will create, if "user B" pings back to "user A", that should happen in the same conversation Id.
My question is how would "user B" will know the conversation Id of "User A". And also how to maintain multiple conversation Id for "user A" (If more users C, D, E get logged in)