Sharing a car would certainly be a good way to share the costs of a car, provided that everybody abided by the agreement on how to split the costs.
It’s not like “sharing a car” so much as it is like car rental in bite-sized portions, and like so many things sold in bite-sized portions, it’s more expensive. If you need a car for a day or more, a traditional rental agency is a better bet.
The ZIPCAR concept seems to be working well in Washington, DC. The Capitol Bikeshares are doing well with more stations (places were bikes can be picked up and left off being constructed almost every month. Each system has contracts, rules and payments that everyone is supposed to abide by. Try Google for more information.
This has been done for several decades, although not formally. They used to call them 'neighborhood cars'. I remember hearing the term many times when I was a teenager. They would purchase a car, splitting the payments or up front cost among 5 to 10 houses in the neighborhood and then set a useage agreement. Needless to say, it usually never ended well.