Regarding the other recommended closed box movies, The Tape is easily in the same niche as The Party. However, Circle is one that I think is from a different niche, almost a sub-genre of closed boxed films that includes those like The Belko Experiment, the Cube trilogy, The Killing Room, The Exam, and others. In these stories, the characters may not know each other very well or at all. And life is almost nothing more than a game of survival. In The Party, they know each other intimately. And their lives are somewhat intertwined in one symbiotic relationship. The attendees of the party are almost different parts of a single person. Perhaps Identity is another comparable movie?

Again, thanks for another entertaining recommendation.