Originally Posted by CompanyOfWolves
I can confirm it does work.
We did just that yesterday. Private Scavenge/Official Dedicated Server with 4 players each with a guest signed-in and it worked.
Since this can be a pain to get if anyone's forming a group and need/can't find a last member I don't mind helping out if I'm free.
i can also confirm this works with guests, i was in his game with him and we all got the achievement, 4 of us each with 1 guest signed in.
I'm online right now looking to do "The Real Deal" I found out a glitch but i need atleast 1 other player to help make things easier.