It's not always clear which companies are preparing to go public. From raising venture capital to confidential filings and mandated silent periods, it can take years of planning before a company lets it slip that it has its eye on a big exit event. EquityZen is a secondary-markets platform that lets startup employees sell their equity to institutional investors before a company goes public. About 125 companies currently work with EquityZen, according to CEO Atish Davda.