you could ask him.
auto friends

@ 1wayne
This has been mentioned before and it does seem to be an inadequacy. Perhaps you could ask your friend for his username via an email. Not slick but should work.
I already did ask him but I wasn't sure if I missed something in order for my friend to be automatically added or if it was just a problem with the site.

Based on what happened to me recently, I think that it only auto-friends you if you list their chess.com username in the referral box.
Because this happened to me as well, and I was auto-friended.
I got invited to join this site by a friend. The email sent by the site said that I would automatically be friends with the person who invited me but my friends list was empty when I went through the membership process.
Also how do I find the person's Chess.com persona through his email? When I got the invite, it just gave me the person's email address but not the ID he uses on Chess.com.