Navigate to Members tab on the top and click on Challenges. Then click on View Challenge for the specific Challenge that you want to check the Leaderboard for. (more information on Challenge feature can be found on the article titled "Challenge")
Navigate to the Leaderboard tab on the right. You need to have created a challenge and have members signed up for it. Once they're done, their name will automatically appear on the Leaderboard as shown above.
Your Leaderboard will reflect the status of the members once the Challenge has begun. Based on your selection of "What completes a challenge" in the Challenge section, you will be able to see the rankings of members on that particular date/time.
Click on the blue settings icon under the Manage tab. The View Challenger option lets you view the profile and background of the member and the Remove Challenger option lets you disqualify someone if necessary.