it kinda goes in this order (kr -> eth -> miles) after those three

kr came into the ESF picture with morally questionable means … under the guise they would be surveillanced but they still ended up being detached from the ESF partly because they don't originate from there. Had no qualms about leaving the ESF whether by their own terms or someone/something else’s — when it did happen, it wasn’t unexpected for the other three to join at the time because of events that occur in the early arc of CTL III. So at that point they all were disconnected from the ESF for an indefinite amount of time
then they meet eth 1 arc later who was in the akheron region presumably sent there by her corp when in reality she was sent by herself. eth didn’t seem like she wanted to go back yet to styx so decided to willingly tag along 🫠the others wouldn’t have known what was going on with ethaniel anyways so more of a reason for him to stick with them
and then the miles feature had them stumbling upon him by pure accident … and … maybe the rest is history i cant share 🤫

important distinction warning they're less of an established group and more of a dysfunctional friend group
they were compelled to stick together because forced proximity in survival situations does that to you — but of course, each of them have their own personal reasons too