John Collison cofounded of one of the most valuable private companies in the country: Stripe. Together, John and his brother Patrick took what seemed like a simple idea - making it frictionless for internet companies to accept payments online - and built a fintech company that once fetched a $100 billion valuation. Though that valuation has fallen to a still-stratospheric $70 billion, Collison and his team are still expanding the company and its offerings - while continuing to shrug off the question of an IPO.