The witness told the court she was blindfolded and strapped to a table while a mystery woman sexually assaulted her under leader Keith Raniere's direction.
A former devotee testified in court that Raniere would make female members paddle each other if they disobeyed him.
Trial starts Tuesday in Brooklyn for the alleged cult leader, who has pleaded not guilty to sex trafficking and other charges.
Mack's plea came just hours before jury selection was set to begin in the NXIVM case.
It started in late 2006, when she attended a two-day introduction to Jness in Vancouver.
Mack is not allowed any contact with members of the alleged Nxivm cult.
The actress is accused of recruiting women who were forced to engage in sex with a cult leader.
The other day, a beautiful woman looked up at me and smiled, "Hi!" Her warm and open energy felt comfortable. "Hi!" I said back. But I didn't know her. She knew Chloe. My weekly visits to her living room TV via Smallville created the illusion of instant friendship for her.