Players only need to log into their Mojang account, click on "Send to a friend" and enter their friend's email address.
"Your chum will receive a code in the mail, along with some simple instructions on how to redeem it. It hasn’t even got to be a “friend” you send the code to: enemies, family members and animals are all allowed to play, so long as they can follow the simple instructions listed in the mail. Send and be merry!" states the developer.
The offer lasts from now through to the 31st January 2014.