I am an enCore MOO user as well as a Moodler. Both applications enable you to see "who is online," but I haven't seen a way to chat with people I see online in Moodle in a sort of Instant Messaging way.
Within MOO, if I were to see that Martin were online, I could send him a message and ask if it were OK for me to join him at his location to talk with him. If he said yes, I could move to his location and we could start chatting. I know Moodle doesn't have "rooms" like a MOO, but is there some way to send a message to someone online at the same time with you and either open a "real-time" dialogue or send a message saying essentially "meet me in the chat lounge?"