TheOptimist, your passengers can agree to share the direct operating costs of a flight, but you must always pay your share. By law.
So if 4 of you want to share a flight, you must pay no less than 25% yourself.
You can put up a notice at the flying club saying something like "Anyone interested in sharing costs to Guernsey and back next weekend?", but you may not advertise such a flight in any other way.
Many people have tried various dodges to get round this over the years, but make no mistake, the CAA's Enforcement Branch take a very hard line over what is termed Illegal Public Transport.
See
http://www.caa.co.uk/docs/1196/20071...PRCampaign.pdf
Edit: And similarly if I ask a mate to take me up in their aircraft for a spin I would be paying a lot more than 50%. It's common courtesy.
It's also illegal!