To get the Crowd Control achievement in State of Decay 2 do the following:
Hordes are shown on your map and mini-map as two green walking zombie silhouettes. Driving around your region will spawn plenty of these frequently. The easiest way to eliminate them is to keep on driving -- as in, right through them. They come in different sizes, seemingly from 3 to 10ish zombies at a time, and you'll know you've eliminated them when they disappear from your mini-map. If you bring a few gas cans you could boost this in one drive.
If any climb on your car you can use your E-brake (B button) to powerslide and throw them off, or drive alongside or into stuff to crush them. If they're on a door where a player is sitting, you can repeatedly press X to hit them with your door til them fall off.