Does the Starfinder society play any long term role in this AP? If my players don’t join how difficult does that make things?
It's no problem if they don't join. The Society is just a motivator to get them started in the early going and a potential patron down the line. If they don't join, then in book 2 the Society can still hire them to follow up on the discovery they made in book 1, seeing them as an interested and reliable outside party.