An NIS+ administrator is anyone who has administrative rights over an NIS+ object. For the purpose of this discussion, administrative rights are defined as create, destroy, and for some objects, modify rights. (See NIS+ Access Rights for a description of NIS+ access rights.)
Whoever creates an NIS+ object sets the initial access rights to that object. If the creator restricts administrative rights to the object's owner (initially the creator), than only the owner has administrative power over that object. On the other hand, if the creator grants administrative rights to the object's group, then everyone in that group has administrative power over that object.
Thus, who ever has administrative rights over an object is considered to be an NIS+ administrator for that object.
In other words, the NIS+ software does not enforce any requirement that there be a single NIS+ administrator.
Theoretically, you could grant administrative rights to the world class, or even the nobody class. The software allows you to do that. But granting administrative rights beyond the group class nullifies NIS+ security. Thus, if you grant administrative rights to either the World or the nobody class you are, in effect, defeating the purpose of NIS+ security.