The business activity of the group includes the marketing, sale and construction of luxury off-plan properties in the Caribbean and elsewhere, and the SFO is asking investors to provide information on any investments they have made.
The online questionnaire asks respondents for the target completion dates of the properties purchased and, if the target date has elapsed, the reason given.
It also asks investors to provide the details of who introduced them to the investment opportunity, whether sales agents, financial advisers or others.
Last week, IFAonline revealed the company had contacted investors admitting problems with missed payments to some investors.
The interest is paid until the completion of the off-plan property the investor has invested in. But Harlequin has admitted to problems making some of these payments.
The SFO is interested in hearing from people invested in seven Harlequin resorts: Buccament Bay in St Vincent & the Grenadines, Merricks in Barbados; Marquis Estate in St Lucia; The Hideaway in the Dominican Republic; Las Canas in the Dominican Republic; Two Rivers in the Dominican Republic and Garapua Beach Resort in Brazil.
Respondents may then be contacted "to obtain a full witness statement for potential use in a criminal prosecution".
Harlequin Property was also the subject of an alert by the Financial Services Authority (FSA), in which the regulator issued a warning to financial advisers about investing clients' money in Caribbean property through Harlequin.
The FSA has also contacted self-invested personal pension providers asking them for details of members holdings in Harlequin.
With the vast bulk of client money now going on to platforms, who really benefits? The client, the adviser or just the platform provider?