With their popularity growing amid high gasoline prices, car sharing services are increasingly turning to sponsorship to promote their offerings and, in some cases, spotlight their eco-friendly positioning. Market leader Zipcar, Inc. reports it is adding 10,000 new members per month, a rate triple the number of customers it was acquiring a year ago. According to Zipcar’s recent survey of new members, 40 percent cited the cost of fuel as their primary reason for using the service.Overall, a...