Mack entered her plea to racketeering charges on Monday, shortly before federal jury selection was scheduled to start in Brooklyn federal court.
The trial is expected to detail sensational allegations that the group, called NXIVM, recruited sex slaves for its spiritual leader, Keith Raniere.
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Prosecutors accused Mack of helping Raniere recruit women to a secret sub-society within the group.
Authorities say the women were branded by a surgical tool with a symbol that resembled Raniere’s initials. Mack has said in an interview that the group emphasized self-discipline and self-empowerment and she likened the branding to getting a tattoo, but cooler.
The defense says the women were never abused.