The concept of a Mastermind group was formally introduced by Napoleon Hill.

In his book “Think and Grow Rich“, Hill described the Mastermind principle as, “The coordination of knowledge and effort between two or more people who work towards a definite purpose in a spirit of harmony…no two minds ever come together without thereby creating a third, invisible intangible force, which may be likened to a third mind”, also known as, the Mastermind.

The Mastermind concept was inspired by Hill’s many conversations with the highly successful businessman, Andrew Carnegie, who attributed his entire fortune and success to his Mastermind group.
After: http://projectlifemastery.com/what-is-a-mastermind-group-napoleon-hill-think-and-grow-rich/