secret society

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An organization, such as a lodge, that requires its members to conceal certain activities, such as its rites of initiation, from outsiders.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A club, fraternal organization, or other association which admits only selected individuals as members, whose meetings and practices are often ritualistic and not public, and which often has the purpose of furthering the social or economic opportunities of its members.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a society that conceals its activities from nonmembers