Slack is one of the fastest-growing B2B businesses of all time. So how can the company continue to innovate in the face of breakneck growth? In this sit-down interview with Fortune writer Erin Griffith, Slack founder Stewart Butterfield shares how he leads when his team (and bank account) are getting larger each week. His biggest struggle: finding the right people. "Every practice that we develop for how to manage becomes obsolete in 60 days," he says.