shared owners
I know of many groups which are ran well, many are not and availabilty is ad hoc..
Some share holders are fortunate that the other members hardly ever fly. The aircraft therefore is available whenever it is wanted. But if the group is active, which is the only way costs are kept reasonable, bookings can be just as hard to obtain at short notice as club aircraft may be.
Sure, if your a shareholder book the aircraft for a week or fortnight and off you go without paying for minimum hours of daily rental. However, when this is the case the other shareholders are sans an aeroplane to fly.
Share holding in a busy group will be cheaper but no more accessible than club aircraft. In a group of not frequent flyers then the aircraft is waiting for you but at a cost. In these not so busy groups getting the non flyers to pay up for maintenance or for the FM immune radio can be a nightmare.