The Photon Chat solution is separate from the in-game communication. This makes your clients receive messages, no matter if they are in a room or in matchmaking. Friends can be found and you can have a user status.
Quick Start and Manuals
Getting Started with Photon Chat
Photon Chat is really easy to use. Check out the demos in the SDK.
The class ChatClient and the interface IChatClientListener wrap up most important parts of the Chat API.
The class AuthenticationValues allows you to set a UserId or use Custom Authentication. Set the ChatClient.AuthValues before you connect. The Chat API has this class in it's own namespace.
Read the readme.txt file at least once.