If you are looking to get this achievement but don't know how to or are having problems getting it, this is the guide for you.
For this achievement you simply have to have a friend join the game, problem is that the games multiplayer achievements are an Ungodly, Broken mess so I will include two ways that can unlock this both of which I have had to use
First you must have at least one person willing to help you (I would recommend having four to get the other achievements as well) and they must be on your friends list. Second (As I have found this to be an important step and not mentioned in the other guides) is to create a match in the Player Lobby as Ranked matches do not seem to unlock it.
After you have done that one of these two steps should work.
1. Have your friend join your match or
2. Send an invite to your friend and have them accept and it should unlock
I hope this helps and if there is any question or improvements I need to make please leave a comment below and happy hunting.