[Decode]

Babyslam, так?

<form action="" class="addPerson">
    <label for="firstName">Имя:</label>
    <input type="text" name="firstName" id="firstName" />
    <label for="lastName">Фамилия</label>
    <input type="text" name="lastName" id="lastName" />
    <label for="month">Месяц</label>
    <input type="text" name="month" id="month" />
    <button>Отправить</button>
</form>

<script>
    var data = {
      "users": [
        {
          "firstName": "Ray",
          "lastName": "Villalobos",
          "joined": {
            "month": "January",
            "day": 12,
            "year": 2012
          }
        },
        {
          "firstName": "John",
          "lastName": "Jones",
          "joined": {
            "month": "April",
            "day": 28,
            "year": 2010
          }
        }
      ]
    };


    document.forms[0].onsubmit = function(e) {
      var inpElems = this.querySelectorAll('input[type=text]'), obj = {};

      [].forEach.call(inpElems, function(item) {
        obj[item.name] = item.value;
      });

      data.users.push(obj);
      console.log(data.users);

      return false;
    };
</script>