You’ll earn this achievement for having Garrus in your active party when you resolve any five missions or assignments. An easy way to grind this out - aside from just rotating your party until all these achievmeents pop - is to just ignore all assignments until you’re done with your first mission on the Citadel, after which go and complete the mission on Therum to recruit Liara. Now you should have all the possible squadmates in the game, and the Citadel should be crawling with assignments. Most of these are incredibly easy, requiring no combat (depending on how you complete them, anyways). Make a hard save, exit the Normandy onto the Citadel, pick the squadmembers whose achievements you need, and complete assignments until their achievements pop. Reload, pick a different squad, do it again. Repeat until they’re all unlocked, then play through the rest of the game with whatever squadmates you choose.