NEW YORK—Robinhood, the trading app that’s brought millions of new investors to the stock market, is buying a company that helps people communicate with the businesses whose shares they buy. Robinhood Markets said Tuesday that it agreed to buy Say Technologies for about $140 million in cash. Say helps publicly traded companies hear what questions their investors want answered, and it helps investors vote at companies’ annual meetings on everything from whether the CEO is overpaid to who should sit on the board of directors. “We share a common goal of eliminating the barriers that keep people from participating in our financial system,” Robinhood Chief Product Officer Aparna Chennapragrada said in a blog post. Say’s Q&A platform allows investors to ask questions of companies, with investors voting on which questions they most want answered. After the earnings release for AMC Theatres on Monday, for example, executives took questions first from …