You may have noticed a few members on Daily Challenge have a "Private Beta Member" stamp on their profile. This indicates the member was a part of Daily Challenge's early testing stage (known in the software industry as a "beta test.") This beta test took place in April of 2010, before Daily Challenge was finished and available to the public. This test helped the MeYou Health team iron out any issues and improve the experience in Daily Challenge.
Since the beta test only happened during three months in 2010, no other members are able to earn this profile stamp. However, there are many other incentives for participating in Daily Challenge, like stamps, points and trees.