Three high-ranking members of the group - Mack, Lauren Salzman and accountant Kathy Russell - are all actively engaged in discussions, a prosecutor said in Brooklyn federal court.
The revelation comes less than a week after Nxivm leader Keith Raniere was hit with child porn charges, and his co-founder, Lauren's mother, Nancy Salzman, pleaded guilty to racketeering conspiracy.
If the three women take pleas, that will leave just Raniere and Nxivm's benefactor, Seagram liquor heir Clare Bronfman, to stand trial - with Mack and the others potentially acting as turncoat witnesses against them.
"I think at the end of the day it will be just Claire and Keith going to trial," Bronfman's attorney Mark Geragos said while discussing possible trial dates.
Raniere is accused of heading a secret master-slave group within Nxivm, called DOS, in which women were branded with his initials and forced to have sex with him - while Mack allegedly served as his right-hand woman.
All of the remaining defendants were in court Monday to be arraigned on a superseding indictment - which included the new child porn allegations against Raniere, as well as various additional charges against the others, including sex trafficking, identity fraud and money laundering.
All pleaded not guilty.
During discussion of the allegations that Raniere had sexual relationships with children and produced kiddie porn, Mack's lawyer gave the 36-year-old a shoulder rub.
Despite the plea discussions, Judge Nicholas Garaufis said he would proceed as if all the defendants were headed to trial, and their own attorneys also said they're still planning for a trial beginning April 29.
The parties are next due back in court on April 4.
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