Go to their profile page (for example, mine is http://www.duolingo.com/lieryan/), and below their name should be a button "Add friend".

If you do not know his/her user name, you will have to find that out. I, myself, have tried to find people's profile even when I knew his/her user name and was not able to find it; Once I had the user name I did searches under discussions as well as under Spanish discussions (which I study) and found the user name if he/she ever made a discussion post. Maybe someone will tell us all how to find the profile page without clicking on his/her name which was on the skills discussion. It might be simple as pie, but I don't know how.

thank you!!!!

I have to work ttyl

kk lylas