The Cult: The Thuggee spend most of their time in a massive underground cave, devoting sweat, blood and tears to Kali, (the Hindu goddess of death). 

Human sacrifice is part and parcel of your membership.

How To Join: It appears that you just need to be uncommonly evil.

An all-consuming hatred of children should help, too.

Real World Equivalent: The Thuggee cult actually existed in India right  up until the twentieth century, where the hooligans were mostly involved in robbing and murdering travellers. Thoroughly unpleasant.