Press Alt and the letter T on your keyboard at the same time and you will see
the Tools menu appear.....from there click Options....
Click Privacy and then click View which is in the middle right side of the
window under Contact lists......you should be able to see who has added you
to their contact list and it should include your friend because they sent the
invite and have you on their list, but since you probably clicked Deny, you
won't show as being online unless you add them yourself......
In that list of people, just look for that persons name in the display name
and put your mouse over whoever it is and wait a few secs.......you'll see
their email address appear. If you don't see a display name in that list
that looks like them, just look through it and pick out which ones in that
list you don't even know who they are and work from there. =)