Send your Flow email address to your Challenge administrator. In order to invite you to the challenge, he or she needs your Flow email address. It's the same one you used when you registered with the service and the one you use when you sign in.
Accept the invite to join the challenge. You'll receive the invite via email and in the notification section of Flow. To accept the invite in Flow: Click your name > Settings > Your profile.
After you've accepted the invite, you're ready for the Challenge!