A new car-sharing program is starting up in San Francisco this month, following the success of similar programs in Portland, Ore., Seattle, Wash., and other American and European cities. Car-sharing programs offer people the opportunity to buy into a pool of cars and trucks that are available for running errands and making other trips. For people who drive only a few times a week or month, car sharing can save money, alleviate parking hassles, and eliminate the need for worrying about maintenance. It can also have environmental benefits by discouraging unneeded trips and keeping down the number of cars clogging the roads, and some car-sharing programs make a point of having fuel-efficient, alternative-fuel, and electric cars. Car Sharing Portland, a private company, has been around since March 1998 and has 800 members.