Non ho mica capito cosa cercavi di fare...
codice:
var admin = ["Paul", "John", "George", "Ringo"];
var name = prompt ("Insert your name"); 
var len = admin.length;
isAdmin = function(){
	for ( var i = 0;  i < len; i++) {
		if (name.toLowerCase() == admin[i].toLowerCase()) {
			return true;
		}
		else {
			return false;
		}
	}
};
if (isAdmin()) {
	alert("Welcome back " + name + " [Administrator]");
}
else {
	alert("Sorry, you don't have privileges to access this page. You must be an Administrator");
}