create a model class that is a singleton See my answer here about making a singleton.
then in your Demo.mxml
[Bindable] public var model:MyModel = MyModel.getInstance();
then in your login form
public var model:MyModel = MyModel.getInstance();
then when you get the response from the service:
model.userVO = userService.getUser();
now in your Demo.mxml, userVO is now populated and usable there.