groupPeople

<script>

function Person(firstName, lastName, age) {
  this.firstName = firstName;
  this.lastName = lastName;
  this.age = age;
  Person.prototype.fullData = function(){
    return firstName + lastName + "\(age " + age +"\)";
  };
}

var people = [
    new Person("Scott", "Guthrie", 38),
    new Person("Scott", "Johns", 36),
    new Person("Scott", "Hanselman", 39),
    new Person("Jesse", "Liberty", 57),
    new Person("Jon", "Skeet", 38)
];

var criteria = "age";
var matchArr = {};
for (var i = 0; i < people.length; i++){
  var key = "Group " + people[i][criteria].toString();
  matchArr[key] = people[i].firstName + " " + people[i].lastName + "(age " + people[i].age + ")";
}

console.log(matchArr);

</script>