Yelp, the review site for restaurants and local businesses, has hired Evercore to help defend the company against an activist investor, who recently called for a board shake-up and potential sale, according to people familiar with the matter.

Hedge fund manager SQN, which owns 4 percent of Yelp shares, released a presentation on Jan. 16, about the company's "significant underperformance," and said that based on its own research "an immediate sale to a private equity firm could yield a $47 to $50 stock price." The shares are currently trading at $37.59.

Yelp co-founder and CEO Jeremy Stoppelman and the board have now tapped Evercore to work with the company and explore the market, but the bank hasn't started running a sales process, said the people, who asked not to be named because the matter is confidential. At the current market valuation of over $3 billion, there are few, if any, "credible buyers," one of the people said.

Representatives from Yelp and Evercore declined to comment.

SQN's report suggests that the company is worth $4 billion or more. But SQN, a technology-focused hedge fund with more than $1.1 billion in assets, is a virtual unknown in the world of activist investing, where Carl Icahn, Bill Ackman and Daniel Loeb are among the most recognizable and influential players. Yelp is SQN's first activist investment.