var UsersList=[
    {
       "User" : {

          "IdUser" : "admin",
          "FirstName" : "mirco",
          "LastName" : "sabatino"

       }
    }, {
       "User" : {

          "IdUser" : "coordinator",
          "FirstName" : "coordinator",
          "LastName" : ""

       }
    }, {
       "User" : {

          "IdUser" : "test",
          "FirstName" : "publisher",
          "LastName" : "Diaz"

       }
    }, {
       "User" : {

          "IdUser" : "work",
          "FirstName" : "ingester",
          "LastName" : "Brown"

       }
    }
    ] ;

<div class="col-md-6" ng-repeat="users in UsersList | filter: {User.LastName : filterSearchLetter}">

$scope.filterLetterUser=function(letter){
       $scope.filterSearchLetter=letter;
       console.log($scope.filterSearchLetter);
    };

todos.filter('startsWithLetter', function () {
    return function (items, letter) {
        var filtered = [];
        var letterMatch = new RegExp(letter, 'i');
        for (var i = 0; i < items.length; i++) {
            var item = items[i];
            if (letterMatch.test(item.User.LastName.substring(0, 1))) {
                filtered.push(item);
            }
        }
        return filtered;
    };
});

Uncaught SyntaxError: Unexpected token .

//iterate through array
<ul ng-repeat="users in UsersList">
  //iterate through Object
  <li ng-repeat=(key, value) in users | filter: {User.LastName : filterSearchLetter}>
     {{key}} : {{value}}
  </li>
</ul>