Have been one of aarp's most effective methods of mobilizing thousands of its members for political action.

The answer to this question is telephone tree
A telephone tree is A phone tree is a prearranged system for activating a group of people to do something through telephone.
As new technolgy developed, they started to change their method into email and newsletter.

An informal structure in a bureaucracy would be something like so-called "office politics". where there's a formal structure set up for the organization, but then an informal subculture as well that has a way of organizing the bureaucracy further. this can be positive because it sets up an added degree of organization, albeit informal, within the bureaucracy.