In about 40 minutes, Cindy Manit will let a complete stranger into her car. An app on her windshield-mounted iPhone will summon her to a corner in San Francisco’s South of Market neighborhood, where a russet-haired woman in an orange raincoat and coffee-colored boots will slip into the front seat of her immaculate 2006 Mazda3 hatchback and ask for a ride to the airport. Manit has picked up hundreds of random people like this. Once she took a fare all the way across the Golden Gate Bridge to Sausalito. Another time she drove a clown to a Cirque du Soleil after-party.