What I've done is go to the person's Profile. Sort of at the top is a place to click on that says 'Add me as a friend'. You'll get a message box if you want to send a short message. Then there's a 'Submit' button under that message box. You'll get a message that says something like "Friend Acceptance Pending" that you need to click OK on. Then you wait for them to be online and respond to you.
On your own Profile is a place to click on 'Friends' and that will bring up the list of people that you are now 'Friends' with and you can email them directly from that.