{   
    //you can define multiple editors   
    propertyEditors: [      
        {
            /*this must be a unique alias*/ 
            alias: "MultiUserPicker",
            /*the name*/
            name: "Multi User Picker",
            /*the html file we will load for the editor*/
            editor: {
                view: "~/App_Plugins/MultiUserPicker/MultiUserPickerEditor.html",
				valueType: "JSON"
            }
        }
    ]
    ,
    //array of files we want to inject into the application on app_start
    javascript: [
        '~/App_Plugins/MultiUserPicker/MultiUserPicker.controller.js'
    ]
}

This is more or less self explanatory. Important in this case to notice is valueType: "JSON". This tells Umbraco what the internal data storage type for this field is. You can find more about this types here. You could use INT, STRING, TEXT, DATETIME or JSON as value type. View is path your editor HTML file and the last line tells where our AngularJS controller is.

Once package.manifest is defined you are already able to use it in Umbraco as custom data type. In Developer section expand Data types and click on Create new.

You should be able to select our new Multi User Picker editor from the drop down. 

Next we need a way to get Umbraco Users to our front end client side code. The way we do that is by creating specialized Umbraco WEB API controller.

namespace UmbracoProject.Controllers {
    public class UmbracoUser {
        public int Id { get; set; }
        public string Name { get; set; }
        public bool Selected { get; set; }
    }
    [PluginController("CustomEditors")]
    public class MultiUserPickerApiController : UmbracoAuthorizedJsonController {
        
        [HttpGet]
        public IEnumerable<UmbracoUser> GetAllUsers() {
            int totalUsers = 0;
            var allUsers = Services.UserService.GetAll(0, int.MaxValue, out totalUsers);
            var retVal = allUsers.Select(user => new UmbracoUser {
                Id = user.Id,
                Name = user.Name,
                Selected = false
            }).ToList();
            return retVal;
        }
    }
}

This controller exposes simple JSON API with one GetAllUsers method that returns list of all users in CMS. We will call this method (endpoint) from our AngularJS controller.

AngularJS controller code:

//adds the resource to umbraco.resources module:
angular.module('umbraco.resources').factory('multiUserPickerResource',
    function ($q, $http, $routeParams) {
        var apiCallsFactory = {};
        //the factory object returned
        //this cals the Api Controller
        apiCallsFactory.getAllUsers = function () {
            return $http.get("/umbraco/backoffice/CustomEditors/MultiUserPickerApi/GetAllUsers/");
        }
        return apiCallsFactory;
    }
);
angular.module("umbraco")
    .controller("multiUserPickerController", function ($scope, multiUserPickerResource) {
        // Fire when page is loaded and Plugin gets pulled in
        Reload();
        // Loads users from actual Umbraco page
        // Then calls Users API and adds missing users if one was added meanwhile
        function Reload() {
            //Bind Umbraco model to our local model
            $scope.Users = $scope.model.value;
            // Get Users from API
            var promiseGet = multiUserPickerResource.getAllUsers();
            promiseGet.then(function (pl) {
                var usersFromWebApi = pl.data;
                // Check if have some values comming from Actual Umbraco Fields value
                // if not just map all users from API to local model
                if ($scope.Users != "") {
                    angular.forEach(usersFromWebApi, function (usrFromApi, key1) {
                        var found = false;
                        angular.forEach($scope.Users, function (usrFromContent, key2) {
                            if (usrFromApi.Id == usrFromContent.Id) {
                                found = true;
                            }
                        });
                        if (!found) {
                            $scope.Users.push(usrFromApi);
                        }
                    });
                } else {
                    $scope.Users = usersFromWebApi;
                }
            },
            function (errorPl) {
                $log.error('failure loading Users from API', errorPl);
            });
        }
        // Map our local model to Umbraco page model to be autmaticaly saved to database
        $scope.SyncWithUmbracoModel = function (users) {
            $scope.model.value = users;
        };
    });

Basically what this code does is, on top of the listing I am registering my custom Angular factory that will be used for calling my custom .NET JSON API endpoint. Under the factory I am registering new controller within Angular application and passing in my factory as parameter (DI). Reload method is called on load, so each time page with our property editor loads we want to automatically show some values. When we defined our valueType in package.manifest as JSON we told umbraco to store values of this field in database as JSON (string) so anytime our controller gets instantiated we will get $scope object trough dependency injection. $scope.model.value is what holds value to be stored to database when the page is saved. When Reload method get triggered it assigns this value coming from Umbraco to my custom local Users model. This doesn't have to be this way necessarily, I could work only with $scope.model.value, but I found it more meaningful this way.     Reload method then calls our .NET endpoint and gets the list of all Umbraco users defined on system. It adds new Users to local model if some users were defined since last page save. Removing of the users is intentionally left out because Umbraco doesn't let you delete any users once defined. Last method from controller simply maps local model back to $scope.model.value to be saved to Umbraco database when page where editor is gets saved. 

Last piece of the puzzle is HTML template and it is defined as follows:

<div ng-controller="multiUserPickerController">
    <div>
        <label ng-repeat="usr in Users">
            <input type="checkbox" ng-model="usr.Selected" ng-click="SyncWithUmbracoModel(Users)" value="{{usr.Id}}" />
            {{usr.Name}}
        </label>
    </div>
</div>