In the features section, one of the features is talking with other players in real time. I'm not sure if that refers to the messages area or if there is another instant messanger feature other than IRC that is built in that I can't find.
If you happen to be at the same location as another player (Which isn't very often), you can choose to "Hail" them using your scanners, which sends them a "Darkness Message" which is the envelope-icon on the header-bar.
Alternatively of course, you can simply click the envelope to send them one if they *aren't* at your location...funny how that works.
To answer the unasked question, yes the mention of that in "Features" is probably a little undeserved and over-exaggerated.