Не ми стигна времето за оформяне на резултата, но крайното решение ми хареса и реших да го споделя в тази тема :)

function Person (firstName, lastName, age) {
  this.firstName = firstName
  this.lastName = lastName
  this.age = age
  this.toString = function () {
    return this.firstName + ' ' + this.lastName + 'age(' + this.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)
]
function groupBy (criteria) {
  var selection = []
  for (var i in people) {
    if (!selection.hasOwnProperty('Group ' + people[i][criteria])) {
      selection['Group ' + people[i][criteria]] = []
    }
    selection['Group ' + people[i][criteria]].push(people[i].toString())
  }
  for (i in selection) {
    console.log(i + ' : [' + selection[i].join(', ') + ']')
  }
}
groupBy('firstName')