I think you should be able to figure it out. It is a social studies question. Which five countries share: a common language, a common system of government (consentual democracy), a common system of laws, a common culture. People, ideas and investment move freely among them. There hasn't been a war among members for over 200 years. The idea of a war now between any members of this group is laughable and perhaps even impossible given the degree of military integration they share. They all fought side by side in WW 2 and most recently in Afghanistan to combat a common foe.
Once you recognize the countries, you will understand why there will be no new members to this club. That's why the Euros are so jealous. Wiki describes it as a "naval agreement" but it goes way, way beyond that. The "naval agreement" is probably just a cover.
http://en.wikipedia.org/wiki/AUSCANNZUKUS