BOSTON—Cort Johnson, 27, is an affable guy who's skilled at promoting his company—a mobile-application start-up he co-founded. But when it comes to socializing, especially with women, he tends to clam up.

"I'd like to be romantic," he sighs. "But how?"

For Mr. Johnson and many others, the answer is "hire a coach."

Hoping to meet some prospects at a holiday party in December, Mr. Johnson enlisted Thomas Edwards, who runs a service called "The Professional Wingman." For a fee of $125, Mr. Edwards accompanied Mr. Johnson to the event and posed as his good pal. As they negotiated the crowd, the wingman alerted his charge to flirtatious types and helped make seamless introductions.

"I love that," said Mr. Johnson.