At the beginning of the Operation Market mission, go forward through the field 
until you find some green bottles on and below some hay bales. Note: This is 
before the intermission sequence with Nicolas is triggered. Shoot and break the
bottles in the correct order then turn around and go back in the direction you 
came from. Walk inside this bush and press Square. A sound and falling confetti
will confirm that the Sheep Squad has been unlocked. You can now use the nearby
sheep as an additional squad.