## Most of the army ends up prone in the square with the circle ##

Assume the circle is in the center of a 15'x15'x15' room. The first occupies the circle, the next eight people occupy the eight 5' squares adjacent orthogonally and diagonally. 

The tenth appears in the nearest unoccupied space, which is 5' above the circle. Since falling in D&D is instant ([well, 500' per round][1]) they fall on the person in the circle and both probably need to make opposed checks to avoid falling prone. The eleventh also appears in this space because due to the simultaneously simultaneous and non-simultaneous nature of turns (this is all happening in the same six seconds, but everyone finishes their turn before anyone else starts theirs) the tenth has already vacated the space due to gravity being grumpy about the whole thing. 

Each additional person who comes through the circle will fall onto the mounting heap, and make an opposed check against whichever person is standing or fall prone.

Even with a bigger room, the space directly above the circle is closer than any square that is two squares away. Depending on how you treat diagonals the 6th person through may be the first to take a fall (since a diagonal square is farther away by traditional measure).


  [1]: https://rpg.stackexchange.com/questions/75954/how-far-do-you-fall-per-turn/109771#109771