Founding is the act of starting something new. If you're a founding member of your school's science club, you were one of the people who got the club off the ground. Well done.
In American history, the founding fathers were political leaders like George Washington, Ben Franklin, and Thomas Jefferson. They were instrumental in establishing American independence from British rule. If you start something such as a newspaper, website, or literary magazine, you're the founding editor. And if you ask people to play music with you and they do, you might just be the founding member of a band.