<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.6.0/angular.min.js"></script>
<script src="https://cdn.firebase.com/libs/angularfire/2.2.0/angularfire.min.js"></script>
<script src="https://www.gstatic.com/firebasejs/3.6.6/firebase.js"></script>

app.controller("CtrlList", function($scope, $firebaseArray){

    var ref = firebase.database().ref().child("users");

    $scope.addUser = function(){
      $scope.users.$add({
        text: $scope.newUserText
      });
    };

<form ng-submit="addUser()">
  <input ng-model="newUserText" />
  <button type="submit">Add User</button>
</form>