Hover over 'Play' in the left side menu of any Chess.com page and click '4 Player & Variants'

Select 'Horde' from the list on the right side:

Standard chess rules apply — stalemates result in a draw, pawns can be promoted, en passant captures are allowed, etc.

White pawns on the first and second ranks can move two squares forward if they have room to do it. Furthermore, pawns that started on the first rank can move two squares up even on their second move.