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S ix years ago, Allison Mack sat on a purple rug filming a segment for her YouTube channel. Bubbly and earnest, she answered fans’ questions with delight.

The “most awesomest” gift she’s ever received? A colorful scarf knitted by a friend. “But really it’s a hug made out of yarn,” Ms. Mack said .

As charming and innocuous as she appeared, Ms. Mack was entranced by an organization that promoted itself as a mentorship program but was on the brink of devolving into a so-called sex cult. Ms. Mack herself would later be accused of recruiting women as “slaves” for the group, Nxivm, branding them and demanding “collateral” of explicit photos and videos.

In April, the 36-year-old actress pleaded guilty to racketeering and racketeering conspiracy and faces up to 20 years in prison. She may be called to take the witness stand in the coming weeks in the trial of Nxivm’s leader, Keith Raniere.