secret_society

salaseura 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 opportunity opportunities of its members.
1879, w:Louisa May Alcott|Louisa May Alcott, Under the Lilacs, ch. 22:

Ben had much fun out of it, assuring the other boys that those were the signs and password of a secret society to which he and Sam belonged.

But these Brothers of Pity were a secret society, known to no man except their spiritual head, who chose them in turn, and not knowing even each other.

Originally a secret society, B'nai B'rith came into the open in 1920 with publication of its ritual.

In the opening scenes of "The Good Shepherd," the hero (played by Matt Damon) is initiated into Skull and Bones, Yale's iconic secret society, by lowering himself naked into a coffin to confess his deepest secrets.