AppWarp supports sending a message in a room/lobby or to a person privately. To send a private message you don't really need to be in the same room he is in. E.g. If you are in a room A and the other person is in room B, you can still chat with him using sendPrivateChat API.
To send chat to a room/lobby, you have to join room/lobby. If you are in a room then you can not send chat to that room only and the players who have subscribed to the room will receive the message.
Please let me know if you have any query, I will be happy to help you.