I am not suggesting that, absent a company, everyone is liable for everyone else, as such is not the case. I'm simply making the point that the statutory strict liability applies to the owner of the aircraft.
I agree that an unincorporated group owning an aircraft will not ordinarily be a partnership, because the group members are not engaged in a joint venture with a view to profit (this assumes that the aircraft is only used privately and is not rented out or used for paid instructional work).