What is the Order of Skull and Bones in the book, Behold a Pale Horse?

The Order of Skull and Bones is a secret group based at Yale University. The group inducts rising seniors and is largely a social club akin to a fraternity. Cooper alleges that the group is actually a secret society which controls the Secret Government of the United States. Membership is not limited to Yale graduates and includes everyone in government, who is knowledgeable of extra-terrestrial activity. This claim is, of course, circular; if membership is not known then how can it be known that a given person is a member of the society?