I didn't know either until...5 minutes ago. If you go to http://www.fitbit.com/user/2S7XW8 you should find me! I haven't found a way besides that little 6 digit user code there. You can find yours by going to your own profile page, if you want to add it to the challenge spreadsheet or share it with us. Let me know if that doesn't work!
Edit: Just change the 6 digit code in the URL to access someone's profile and add them as a friend directly. I haven't found a username search option anywhere on FitBit. :/