This achievement can be hard or it can be easy, First off as its said above you do need 7 people playing with you to get the achievement 6 friends and you.( NOTE! All seven people must finish the game for the achievement to unlock at the end of the game, make sure you have everyone your playing with on your friends list! )
There are a few ways you can get this achievement each way may be hard or easy depending on your luck.
You can have a party of 7 people and hope that when you join a game no one gets booted out or cant connect to the game which when i did it happend many times.
You can also while in game send friend requests to everyone on your team which you may have luck and theyll accept it in game, i only suggest doing this with atleast a party of around 4 guys or if your in a party of 7 and one of you get lagged out of the game send a friend request to your other teamates in hope that theyll accept for your 7.
You can also play a free for all match which will also unlock the achievement just by haveing everyone search one game mode at the same time and hope you get pushed into one game, as long as your all on each others friends lists you'll see it pop at the end of the game. Can also add anyone from the game to see if you can get 7.
As long as its a ranked match can be team or no teams either way if you play with 7 friends it will unlock at the end of the game, Hope this helped i know this one can be annoying to obtain took me a week just to pull it off and it was my last one.