Complete any mission with two other players and this will unlock.
If you each select the same chassis you will also unlock:
The A Few of the Same achievement in Livelock worth 31 pointsComplete a COOP Mission with 3 players using 3 of the same class
If you each select one of the three chassis each (Hex, Catalyst and Vanguard) you will also unlock:
The Few's Company achievement in Livelock worth 25 pointsComplete a COOP Mission with 3 players using Hex, Catalyst and Vanguard