how to find specific opponent

Can anyone help? A friend and I joined chess.com so that we could play each other on-line. Neither of us can find any method whereby we can start a game between ourselves, there is no obvious means of doing so on the site, and no indication as to how to do so. Could someone point me in the right direction please, and does my friend actually have to be online at the moment I start, or can I just start a game about which she is notified when she comes on line. Many thanks ahobbs1

If you know their username, go to their profile page (http://www.chess.com/members/view/[username]) and, on the side, find where it says "Challenge to Play."  Click on that, fill in the options, and click the "Create new game" button.