Here is the description for the trap:
This hallway contains an elevator trap, triggered when at least 400 pounds of pressure is applied to the 10-foot square section of floor in the center of the hall... A party of adventurers moving in close formation down the hall is certainly heavy enough to trigger the trap. (Curse of Strahd, p. 74)
Medium-sized creatures occupy a 5 foot square; they cannot occupy the same space per PHB p.191/SRD p.92):
You can move through a nonhostile creature’s space... another creature’s space is difficult terrain for you. Whether a creature is a friend or an enemy, you can’t willingly end your move in its space.
Is "close formation" an aspect of the rules I have missed? Given further description of the trap in the text, it seems as if the design of the adventure anticipates more victims than I'd assume:
All creatures inside the elevator (including the unconscious ones) must roll initiative...Each creature inside has one turn to act before the compartment comes to a stop. Their initiative rolls determine the order in which the occupants act...They might search for a way out, wake sleeping party members, cast spells, or take other actions.
How many creatures can be victimized by this trap?