When two or more people are logged into a map at the same time, a small co-presence indicator appears in the top right corner of the screen:

Rolling over the co-presence indicator opens a dialog box that let you learn more about, and, if you like chat with the other people listed .

(1) The Chat link works in the same way as the Comment button and also displays your message (temporarily) as a chat alert on the main map screen.

(2) Each person's name is displayed as a link, which, when clicked, opens the Map Community list with their profile highlighted; from where (if they allow this) you can send them a private direct message using the Contact>> link and/or Follow>> their public activity stream.