You have to have your friend's gamertag and have them link an Xbox One to their gamertag during that time period. That is what I got from reading all the info. I just broke down and got an Xbox One last week, and got the email today.
Hey guys,
Just got this in my Inbox today!!
Experience Xbox One with friends, earn 25,000 Rewards Credits.
Invite your friends to experience the next generation of games and entertainment, and you'll both earn 25,000 Rewards Credits! Simply refer your friends; if they pick up an Xbox One...