http://www.wow-europe.com/en/info/fa...itafriend.html

From what I can tell if you have a copy of the game you can give up to 5 people at a time to a trial vertion of the game which they can upgrade to the normal game but to upgrade to the full vertion of wow you need to buy the game (as far as I can tell) but when you do your account that invited the other gets 1 month free. This will save you some of the monthly costs on your main account and also let you try multi boxing before you buy your copys of wow.