How do i add a friend on psn?

Im trying to add my classmates on psn so we could play blackops. I got their psn and i wrote it down. But it always says: "The online ID you entered has not been registered. Check that you are using the correct online ID." I tried retyping it and actually typed it the same way my friend wrote it. I checked twice to see if i got any miss spelled words. But it said that same sentence! Can anyone help?