It is the Indian Railways. It has presently more than 1.6 million employees, civilian. Insofar as military organizations are concerned, the largest is the US Military (including reserve), with 2.8 million members. (However, if China's numbers are to be trusted, the size of their military is listed at 7.5 million members, in total.) Walmart reports 1.9 million employees. The Red Chineese Army reports 2.3 million strong. Jehovah's Witnesses report (confirmed by several sources) in excess of 7.5 million active ministers.
USA department of denfense
Marc antoni
Fifa (soccer)
The united Nations
It's Indian Railway
It is the largest information technology in this world.
United Nation (UN)
The largest collection is probably in the hands of the DeBeers diamond organization.
The largest student organization of the world is the Indian nationalist student organization called ABVP or Akhil Bharatiya Vidyarthi Parishad with 17,80,802 registered, dues-paying student members. (Nov 2010)
The World Health Organization is one of the world's largest organizations, and it is an organization because it is a group of people who are united around one common goal, and who work to pursue that goal.
The American animal cruelty/rights prevention organization, the People for the Ethical Treatments for Animals (PETA) is the largest in the world.
National Geographic Society
BRAC (formerly the Bangladesh Rural Advancement Committee) - the largest non government development organization on the planet. It is based in Bangladesh. India's largest NGO is "Help Age India".