Millennial expert Lindsey Pollak says she can teach companies a thing or two about young workers. First, they expect work to be meaningful. Second, they crave frequent feedback. Third, they despise voice mail.

People born in the 1980s and 1990s now make up the largest single generational group in the workforce, and managing them has become a source of agita for big businesses such as Oracle Corp., Goldman Sachs Group Inc. and Coca-Cola Co.

Millennial issues also have become a source of income for a host of self-anointed experts who say they can interpret young workers’ whims and aspirations—sometimes for as much as $20,000 an hour. Oracle, Red Robin Gourmet Burgers Inc. and Time Warner Inc.’s HBO have retained millennial advisers to stem turnover, market to young people and ensure their happiness at work.

“There is somewhat of a disconnect between young people, their hopes, goals and expectations, and what companies think young people want,” said Ms. Pollak, who has advised Estée Lauder Cos ., Hartford Financial Services Group Inc. and LinkedIn Corp. “I see my role as a translator.”

Intergenerational consulting barely existed a few years ago, but these are boom times. Source Global Research, which tracks the consulting market, estimates that U.S. organizations spent between $60 million to $70 million on generational consulting last year. More than 400 LinkedIn users globally list themselves as a “millennial expert” or “millennial consultant.”