How do I add someone to my friends list?

In the user profile, there is the Buddy List section, under which there is a heading Buddy actions and the action Add user to my buddylist, where user is the user name of the member at whose profile you are looking. Clicking this will result in that user getting an eMail request for you to be added to their buddylist. Provided they approve it, they will appear on your buddy list, and you can then see who of your buddies is on-line. You'll also appear on their buddylist.