Here's a review and reassessment by the late Roger Ebert that appeared in May 1997:
The dinner guests arrive twice. They ascend the stairs and walk through the wide doorway, and then they arrive again--the same guests, seen from a higher camera angle. This is a joke and soon we will understand the punch line: The guests, having so thoroughly arrived, are incapable of leaving.
Luis Bunuel's "The Exterminating Angel" (1962) is a macabre comedy, a mordant view of human nature that suggests we harbor savage instincts and unspeakable secrets. Take a group of prosperous dinner guests and pen them up long enough, he suggests, and they'll turn on one another like rats in an overpopulation study.
More: http://www.rogerebert.com/reviews/great-movie-the-exterminating-angel-1962
More about the event: http://www.albany.edu/writers-inst/webpages4/programpages/cfs.html#angel