My neighbour is into car sharing for two reasons - cost benefits and poor local infrastructure (Devon). As a result he has investigated the possibility of sharing ownership of a car rather than just having regular shared journeys.
The benefits are proving to be quite impressive for him as he has found a couple who have agreed to share the cost of owning a car. They both work locally and do not need the car during the week, he is retired and is not fussed about having the car at the weekend, so it works well for them both.
He found a comprehensive, ready drafted, legally binding contract suitable for the UK. Apparantly the company also do contracts for motorbikes and campervans. If you want to find out more the company is called
Sharing Contracts.
I hope this is useful to anybody looking to share on a more permanent basis.
regards
David