Because this is not a chat room, you must go offsite to do this; any personal contact is dependent on the recipient's settings. You cannot contact someone who chooses not to be contacted.
To see if anyone else allows email or IM, you need to have at least one enabled on your settings. Click on the MY ACTIVITY tab at the top of the page and then on Edit My Preferences. Check around and you'll find it. Then click on the other user's avatar to get to his or her profile page to see if IM or email is allowed.