What is US' 'Gang of 8', the secret group Trump briefed before strikes on Iran?

Before launching strikes against Iran, US President Donald Trump briefed a small and highly secretive group of Congressional leaders known as the "Gang of 8". It includes US' Speaker of House of Representatives, Minority Leader of the House, Majority and Minority Leaders of the Senate, House Intelligence Committee Chair and Ranking Member, Senate Intelligence Committee Chair and Ranking Member.