"The Association of British Insurers clearly states that car-sharing won't affect the insurance of their members so long as a profit isn't made:
"Giving Lifts - All ABI motor insurers have agreed that if your passengers contribute towards your running costs your insurance cover will not be affected, as long as lifts are given in a vehicle seating eight passengers or less. This agreement does not apply if you make a profit from payments received or if carrying passengers is your business." (Source: ABI website, 2005)
The ABI represents around 400 companies, transacting 94% of the business of UK insurance companies. Drivers should check with their insurance company as terms and conditions may vary between insurance providers over time. "
http://www.carsharegm.com/faq.aspx#insurance
Of course it does no harm to clarify the situation with your own insurance company if you are at all worried