The Company of Strangers

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Complete a co-op mission with at least 5 different players

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This achievement is very straightforward and self-explanatory. When you boot up the Co-Op Mode from the Main Menu of the game, simple press X to start searching for a teammate. Take note of the gamertag when the player joins, so you do not accidently end up completing another mission with him when you go to search the next few times.

You will need to complete any mission all the way through in order for it to count towards the achievement. The second mission is, without a doubt, the quickest and easiest of the five to complete, so set the preferred mission to this before searching for a partner.