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Yammer founder David Sacks has joined Zenefits as its first chief operating officer and will be making a “major investment” in the company as well.

In a press release, the San Francisco-based company that offers cloud-based human resources services, didn’t disclose the size of the investment but said Sacks will join its board of directors.

“David is widely acknowledged as one of the best leaders and managers in tech,” Zenefits chief executive and cofounder Parker Conrad said in a press release. “My cofounder, Laks Srini, and I couldn’t think of a better partner to help us build Zenefits into a company that will transform the way businesses across the country manage their employees and HR.”

Sacks is a relatively big name in Silicon Valley, and it’s interesting to see him take a seat in the sidecar of a company rather than take the helm.

Most recently, Sacks founded Yammer, which Microsoft bought in 2012 for $1.2 billion. Before that, he was COO and product leader of PayPal. In addition, he’s become an influential angel investor, backing companies like Facebook, Uber, and Airbnb.

According to the press release, his investment in Zenefits “represents his largest investment to date in a private company.”

Sacks joins a company that’s been growing fast, but also has been hitting some speed bumps.

The company has drawn regulatory scrutiny in Utah, where it may or may not have been banned because it offers its basic service for free.

Zenefits said it serves 2,000 businesses across 47 states. And it says its user base will grow 1,600 percent in 2014. The company now has 420 employees and plans to hire 1,300 more over the next three years.

“I have never seen a software-as-a-service company grow its revenue this fast,” Sacks said in a statement. “By comparison, it took Yammer three years to achieve what Zenefits has done in one.”

Prior to Sacks’ investment, Zenefits had raised $84 million in backing from investors that include Andreessen Horowitz, Institutional Venture Partners (IVP), and actor and investor Jared Leto.

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