angular: form input to update section of nested json file

Using angular I want my form input to update the 'replied' section of a nested json file. When the form submit button is clicked I want the function add() to push the users first name, last name, and message to replied section of the json file.

app.js:

    $http.get('/app/javascripts/data.json').success(function(responsetext){
           angular.fromJson(responsetext);

                $scope.activity = responsetext.activity;

                $scope.replied = responsetext.replied;
                //console.log(responsetext.replied);

                $scope.admin = responsetext.admin;
                //console.log($scope.admin); 

                $scope.menu = responsetext.menu;
                //console.log($scope.menu); 
             //console.log($scope.replied);
            })
            .error(function (responsetext) {
                 console.log("Error retrieving user data.");
        });

      $scope.showAdd = function(ev) {
        $mdDialog.show({
          controller: DialogController,
          template: '
<md-dialog aria-label="Mango (Fruit)"> 
<md-content class="md-padding"> 
<form name="userForm"> 
   <div layout layout-sm="column"> 
       <md-input-container flex> 
            <label>Contact Name</label> 
            <input ng-model="user.contactName"> 
       </md-input-container></div> 

      <md-input-container flex> 
          <label>Message</label> 
              <textarea ng-model="user.message" columns="1" md-maxlength="150">     </textarea> 
      </md-input-container>
</form> 
    </md-content> 

<div class="md-actions" layout="row"> 
     <span flex></span> 
          <md-button ng-click="answer(\'not useful\');"> Cancel </md-button> 
     <md-button ng-sumbit="add();" class="md-primary"> Send </md-button> 
</div></md-dialog>',
          targetEvent: ev,
        })

data.json:

"replied":[
      {
         "what":"Did You Study?",
         "who":"Keith David",
         "when":"6:45pm",
         "notes":"I'm on my way."
      },
      {
         "what":"Movies Saturday Night?",
         "who":"Kim, Scott, Jennifer",
         "when":"4:31pm",
         "notes":"Wish I could go, but I'm out of town this weekend"
      },
      {
         "what":"Studying Abroad Next Semester",
         "who":"Sandra Adams",
         "when":"12:22pm",
         "notes":"You should do shopping on Champs-Élysées!"
      },
      {
         "what":"Birthday Gift",
         "who":"Trevor Hansen",
         "when":"5:00pm",
         "notes":"Let's give her a surprise party at her favorite resturant!"
      },
      {
         "what":"Grocery Shopping",
         "who":"Lamman Davis",
         "when":"4:35pm",
         "notes":"Don't forget the salas, cheddar cheese, and we are out of almond milk. Thanks"
      }
   ],