From street level in Palo Alto, California, the headquarters of Bill Nguyen’s new startup appears to be an abandoned storefront: A sign on the door instructs you go to elsewhere, and the windows are covered with brown butcher paper.

But those permitted to enter find a bustling high-tech operation. Dozens of energetic young people work on long tables loaded with computer equipment. It’s like a black and white photo exploding into vibrant color, reminiscent of the scene in The Sting where a nondescript door leads to a busy and luxurious gambling joint.

The stealth ends. Nguyen, a fabled serial...