"This is very simple: the picture shows the member you need to make up the Combo: if you hire all of the shown characters, your team will benefit from the Combo Option and thus get its bonuses. Obviously, if you don't hire them all you won't get any bonus, not even a partial one, but that's not all: to get the bonus the Combo Characters must be together and actively co-operate to accomplish something with the rest of the team. So you won't get any bonus if they are apart, even a single one of them is unconscious, or if they are together but away from the team (although in this last case they would still inherit their own bonus)."