Details for how to recruit Garrus.
One of the few optional characters in the game, Garrus can be recruited after meeting with the Council for the first time. In the chat with Anderson and Udina after the aforementioned meeting with the Council you’ll be encouraged to talk to Harkin in Chora’s Den. Do so and Harkin will eventually tell you that Garrus was snooping around near the Med Clinic.
Head up the Med Clinic and sure enough, you’ll find Garrus, who is also taking it upon himself to seek out information on Saren. Help him fight off the thugs threatening Doctor Michel, then accept his offer to join your squad.
You’ll unlock the Turian Ally achievement for completing five missions and/or assignment with Garrus in your active party.
ME1: Complete 5 missions with the turian squad member