Be-aware that as of February 2016 the online for this game is still popular with randoms and boosters alike so you may have some conflicts when you are trying to go for this achievement.
The description for this achievement is misleading. When it says "Six Friends" it means you will need at least 6 other people "excluding" yourself to help you get the achievement. So that means including yourself you will need 7 people. The problem though with this game is that some game modes do not like odd numbers starting up Ranked Matches so as a precaution I recommend you get an 8th player as that should start up the Ranked Match immediately.
So once you have your group of people/friends that are willing to help you get this achievement, you all first need to return to the XBox 360 Dashboard and send Friend Requests. If you do this whilst you are in the game, it won't count and you will void the achievement.
Note: This game is Backward Compatible on the XBox One console, but the Friend Requests will not work via the XBox One Friend Feed. You must send an individual Friend Request on the XBox 360 console. If you are playing this game via the XBox One consoles Backward Compatibility, but you have no access to an XBox 360 console, you will have to send the Friend Requests first, then once they've been approved, reload the game again for the achievement to count. Another possible method around this is to send the Friend Requests whilst playing on a different XBox 360 game that you are running via the Backward Compatibility.
Double check and triple check that you have everyone in the group on your friends list and then when everyone feels comfortable, load up in the game and follow the procedures of getting into a group, searching for a Ranked Match and then getting into a game.
As soon as the match loads up you will get this achievement instantly. You can then either work towards other achievements or quit out if this is the only achievement you need in the game.
Good Luck!