Sorry - I realize there is a ton if discussion on challenges but I didnt find any information on how you sign up for a challenge. This is the first I've heard of these challenges but I just joined Insider today.
Cinderrules, As you'll see from the thread bejacob directed you to, the fastest and most efficient way to enroll in a challenge is to call the Rewards Customer Service line at 1-800-321-7396. They will be able to give you details as well as enroll your account if you are eligible. If you prefer, you can message me with your account details so we can have our team check your account for eligibility.