I'm not sure I understand your question. I'll do my best to respond.
allow([...actions...]) is specific to the Controller that is executing. If you place it in the App controller it will apply to all actions named "add". It's best therefore to keep it in the currently executing controller.
If registration is in the add() method of your users controller, you want to add $this->Auth->allow(['add']) to the beforeFilter function of the users controller and that should allow unauthenticated users access.
Hope that helps.