With the multiplayer update, the ability to message players that are in single player hunts was removed. It is hoped that this featured will be returned to the game. 
How does the in-game chat work?'
Chat only works in multiplayer. Press [Enter] to open the text box. Type the message and confirm with [Enter] to send the message to all players on the map. Pressing [Enter] without a message will close the chat box.
- "/w <player name>": Whisper a message to the targeted player. This message will not be shared with the rest of the players on the server.
- "/r": Whisper a reply to the last player to send you a whisper.
- "/a": Send a message to all
- "/t". Send a message to members of the same team. This can be useful in competitive situations when you don’t want other teams to know your plans. Team messages have a different color than regular messages which all players can see. (It is blue/green)
- "/players": Show the names of all players on the server.
- "/clear": Removes all text from the screen.
- "/afk": Set your status to “Away From Keyboard”. This is only visible to other players if they have the “Show player names” option turned on. AFK will also come up next to players' names on the map automatically if they are inactive for a few minutes.
The Tab key can be used to quickly switch between send modes.