Globally available Angular functions using $scope variable

Question

I'm dealing with a simple problem in AngularJS regarding the visibility of functions between controllers and the $scope and $rootScope variables.

To clarify, I have a couple of functions in several of my controllers that I would like to make available in all of them (for the sake of code clarity and avoid duplicating functions).

I already have functions using $rootScope and services for other functionalities, but I can't find the proper way to address this case.

This is one example, one simple function to toggle an accordion list (Source: http://codepen.io/deinerson1/pen/gxnzj)

// Toggle groups
$scope.toggleGroup = function(group) {
    if ($scope.isGroupShown(group)) {
        $scope.shownGroup = null;
    } else {
        $scope.shownGroup = group;
    }
};
$scope.isGroupShown = function(group) {
    return $scope.shownGroup === group;
};

In this case, how can I make this function (that is using $scope) available to all the controllers that may want to use it?

As a summary:

What is the proper way of making functions available globally, including those using $scope variable?

1. If I use a function inside $rootScope, how can I access/handle the $scope variable that is being used inside that function?
2. Exactly the same question if I use a service.
3. And last but not least, what might be the correct/best way of handling this situation if the previous two options are discarded?

Also, to have another example and avoid getting stuck with the simple accordion thing:

$scope.cancelModal = function() {
    $ionicHistory.nextViewOptions({
        disableBack: true
    });
    $ionicViewSwitcher.nextDirection('back'); // Go back effect
    $state.go('app.main');
}