You press "Add Contacts", enter the email address, just as you add anyone else!
The only difference is you can't play games with them, or use a lot of smiles, send nudges and winks, and a few other things. That's why I use Y!Messenger for my few Yahoo Friends, and MSN with the others. It just isn't as FUN with Yahooers, if you get what I mean, on Yahoo.
