The Wikipedia Zero program was discontinued. See T187716 and for details.
The zeroadmin user group provided user rights to edit the custom and protected Zero: namespace. The Zero: namespace was intended to house per-carrier JSON blobs in support of the re-architecting of ZeroRatedMobileAccess, an extension that worked in concert with the MobileFrontEnd extension to enable mobile access, free of data charges, to Wikipedia in developing countries under the Wikipedia Zero initiative.
The settings stored in the Zero: namespace affected whether users in the developing world would be provided an indication of whether served content is without data charges, providing attribution to participating carriers.