Mack - who is facing charges including sex trafficking, identity fraud and money laundering for her alleged role in running a slave ring within the Albany-area organization - is scheduled to change her previous not guilty plea in Brooklyn federal court at 11:30 a.m.
Prosecutors accused Mack, who starred in television show Smallville, of helping Raniere recruit women to a secret sub-society within the group.
Actress Allison Mack attends an event on July 29, 2010 in New York City. Mack is scheduled to be sentenced on September 11 this year.
Lauren was also a "master" in DOS - and has admitted to keeping her own personal slave locked in a room for two years - while Nancy confessed to hacking into emails of the group's perceived enemies and destroying evidence that could be used against it.
"I was wrong. I now realise that I and others engaged in criminal conduct".
The trial of the remaining defendants, including Raniere and several associated people, will begin later this month. Bronfman fainted while in court March 27 after a judge asked her about reports that lawyer Michael Avenatti was representing her after his recent arrest on extortion charges, reported CBS.
Women were reportedly forced to recruit others in a sort of perverse pyramid scheme.
Many of the DOS members were branded with a cauterizing pen during a process that took "20 to 30 minutes". The symbol itself was a combination of both Raniere's initials (KR) and Mack's (AM).
Self-help guru Keith Raniere supposedly created NXIVM as a multi-media marketing company "to raise human awareness, foster an ethical humanitarian civilization, and celebrate what it means to be human". After composing herself, Mack apologized to people she felt have been victimized by Nxvim. She said in court she had become lost and wanted to find a community. Some of these photographs included close up pictures of their private parts. The group heavily attracted women, including a number of high-profile actresses like Mack, her Smallville co-star Kristen Kreuk, and Battlestar Galactica's Nicki Clyne. All have pleaded not guilty.