Must be done on Escape with 3 people.This achievement is known to be buggy and may not unlock on your first attempt.
While you progress through the level you’ll come across this decontamination room (refuge shower) where their will be 3 separate doors leading into showers.
All 3 party members must shower in the same room by pressing the button on the wall.
Best bet is partying up with some friends or trying with randoms.
I got mine with random players. I played as Keegan and used he’s ammo ability to lure the randoms into the shower room with me.
Edit: You can find the decontamination rooms in these maps:
‘The Hunters’ (Challenge map)
‘The Hive’ (Past hive)
‘The mines’ (Past hive) Note- The map 'The Gauntlet' does not have a shower.