The trial is expected to detail sensational allegations that the group, called NXIVM, recruited sex slaves for its spiritual leader, Keith Raniere. Authorities said the women were branded by a surgical tool with a symbol that resembled Raniere's initials.
Prosecutors accused Mack of helping Raniere recruit women to a secret sub-society within the group. Mack is best known for her role as Chloe Sullivan on the series Smallville. The character was a close friend to Superman on the show.
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 said the women were never abused.