Guo Wengui, a wealthy Chinese businessman, sat in the sun room of his apartment on the 18th-floor of the Sherry-Netherland Hotel on New York’s Fifth Avenue. With him were four officials from China’s Ministry of State Security, whom Mr. Guo had agreed to meet.

For many months, Mr. Guo, from his self-imposed exile, had been using Twitter to make allegations of corruption against senior Chinese officials and tycoons. During the hourslong conversation, the officials urged him to quit his activism and return home, after which...