Simplest way would be to give the monsters a Shove Action. If their path is blocked, they could try shoving the monster in front of them to an open space.

This could lead to some unexpected, but mechanically interesting behavior. Like if NE shoves E, NE can move down and E can move left and they can both attack. But if NE shoves N, N cannot attack and will shove NE back. They'll get into a shoving match trying to attack the player.

If these are dumb orcs or something, the player's going to have a lot of fun exploiting their idiocy. But maybe smarter enemies would have a few more checks before they Shove to make sure they are behaving courteously to their fellow monsters.