I received this achievement after recruiting Rex, without recruiting Cass. I have met her, and she gave me the quest that starts the process of recruiting her, but I haven't actually finished the quest line and recruited her yet. Just thought I would report it.

Lucky you! I recruited all companions... had them all assembled at the Lucky 38... still no achievement! I made sure I kept each one with me for a while before recruiting the next one and even went back to previous saves and re-recruited.
Still no achievement... so feel lucky that it glitched in your favor.