This is a storyline achievement, is not missable (with one possible exception, explained below in the spoiler section), and happens during your mission to Palaven's moon.
If you haven't gotten there yet, you get assigned this mission during your visit to the Citadel, which follows the stop on Mars, which in turn immediately follows leaving earth, so this is a very early-game assignment.
*****SPOILERS BELOW*****
On your mission to Palaven's moon, you're charged with finding a replacement for the killed Primarch. On your way to find him you run into our old friend Garrus Vakarian. He laments a bit about how hard it will be to leave the battle for his own homeworld and I BELIEVE (though I can't remember exactly) you get an option to take him with you or let him stay and fight on the homefront... but if you do take him aboard (and why wouldn't you?) the achievement will pop as soon as he agrees.