An immensely powerful organization that has supposedly has members all over the world. Made up of C.E.O.s, politicians, geniuses, captains of industry, gangsters, etc. People with the power and money to accomplish great things, but also without the moral obstructions to reach these goals.
But due to the nature of their methods, and their agendas, it's membership isn't open to the public. The identities of these members are also largely unknown, even to each other. This is to ensure it's effectiveness, and that no "branch" of the Tree is compromised.