Zenefits, which provides human-resources software and acts as a health-insurance broker for small employers, wouldn't exist without the law, said Parker Conrad, the firm's chief executive officer. Since the Patient Protection and Affordable Care Act's inception in April 2013, the San Francisco-based company has grown to more than 900 employees. That makes it the largest firm among dozens that have sprouted in the law's wake, according to PricewaterhouseCoopers, which issued a report on the trend this week.

The health law, which took full effect in 2014, represents the most dramatic change to the health system in 50 years. Entrepreneurs, including some from within President Barack Obama's administration, have founded companies that target employers, health insurers, hospitals, doctors and consumers looking to navigate new requirements and possibilities.

"There's a lot of opportunity for new market entrants," Zenefits' Conrad said. "The ground is literally shifting under the feet of the incumbents."