On a Saturday afternoon in November, Louis Meza made plans to meet a friend and fellow cryptocurrency enthusiast for drinks at Ruby Tuesday in New York City’s Times Square. “We’re going to have a great time,” Meza said in a text. “And by the way, I’m putting you in an Uber car at the end of the night.”

A former salesman at the food-delivery service Grubhub, Meza is described by friends as gregarious and courteous—a 35-year-old poker buff with an entrepreneurial streak and an appetite for risk. The two men had known each other...