Please don't use the $rootScope to define any function - it's really bad practice. Remember that $rootScope is a global variable after all.
You can get the $rootScope via service like this:
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$rootScope exists, but it can be used for evil
Scopes in Angular form a hierarchy, prototypically inheriting from a root scope at the top of the tree. Usually this can be ignored, since most views have a controller, and therefore a scope, of their own.
Occasionally there are pieces of data that you want to make global to the whole app. For these, you can inject $rootScope and set values on it like any other scope. Since the scopes inherit from the root scope, these values will be available to the expressions attached to directives like ng-show just like values on your local $scope.
Of course, global state sucks and you should use $rootScope sparingly, like you would (hopefully) use with global variables in any language. In particular, don't use it for code, only data. If you're tempted to put a function on $rootScope, it's almost always better to put it in a service that can be injected where it's needed, and more easily tested.
Conversely, don't create a service whose only purpose in life is to store and return bits of data.
UPDATED ANSWER
A_J, I am not sure what's the scenario of your application. I would need to see more code to give you a better answer, but please take a look at the code below. Maybe it can give you some idea how you can design your code to access $rootScope inside a service.
Press the button "Log me In" and take a look at your console. Working example here (http://jsbin.com/olOyaHa/1/)
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