As you progress through the story, you'll pick up a few new faces. The last one to collect is Shindo. Just make sure that you've selected each person to be on your team. This must be at points where the game forces you to choose which people you want. As soon as you can select Shindo, you can get this achievement. It pops right after you start the mission with the last person you need for the achievement.
Squad members are Big Bo (you start with him), Charlie, Rachael, Faye (who you collect after you slide down the Sea Wall and escape the huge white robot whose name escapes me), Cain (the robot sidekick, you get him when you link up with the French team) and Shindo (who you get when you link up with the Resistance).
KJ
EDIT: You must MANUALLY add each team member to your party. If they are put there by natural story progression (like how you already have Big Bo right at the start), it does not count toward the achievement. You must select them yourself in the character menu that pops up from time to time.
KJ