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A former member of a cultlike group called Nxivm offered the first graphic details on Tuesday about how women were manipulated into becoming “slaves” who provided sexual favors to its leader.

The testimony of the former member, a 32-year-old British woman identified only as “Sylvie,” came on the first day of the racketeering and sex trafficking trial of Keith Raniere, Nxivm’s former leader, in federal court in Brooklyn. It gave jurors a glimpse into a group in which prosecutors say women were branded and humiliated.

Sylvie said she had been involved with Nxivm for about 13 years, taking self-help courses, when she was invited to join a secretive sorority within the organization.

But to join, Sylvie said, she first had to prove her dedication by providing “collateral” to the woman who had invited her, Monica Duran, in the form of nude photographs and a stamped letter to her parents saying she was a prostitute.