How can I invite someone I don't know from a past yahoo fantasy football league?

I had a private league, we needed one more guy, so we made it public, and this guy joined up, and was very competitive and did lots of smack talk, plus he likes the Browns and I'm a Bengals fan, so it added lots to the overall league, bantering back and forth. For the life of me, I can't figure out how to get a hold of the guy and invite him to my new league.
How can I invite someone I don't know from a past yahoo fantasy football league?
if he kept email hidden you cant