To play with friends each friend would have to own a copy of the game. Its a good excuse to tell them about gog though and get them their own accounts if they don't already have them. Then to get a CD key you just need to email gog support and have your friends do the same. They normally fill CD key requests in 2-3 business days.
Hey thanks, I will definitely have to get my friends to purchase this game then so we can play together.