After you complete a round of Endless Run Mode, in the screen after you die, there will be a button at the bottom of the screen that says "Challenge Friends." Hit this button and select the names of friends who you want to send the challenge to and click the check mark to confirm. You can select as many as you want, you just need a cumulative total of 25 challenges sent (they can be to the same friend, to different friends, or any combination thereof). This can be done across multiple rounds. The friends you are sending the challenges to do not even need to have this game or even Windows 8.