MLS & Bookworm are quite right, I was responding more to Maz's idea of a contribution.
I like to take people along for the ride but it can be an issue if the person is unaware of the risk. If you take someone on a potentially hazardous activity you have a duty of care. Just look at the canoeing and hill walking incidents that have occurred within education.
While you can to an extent cover yourself with a "blood chit" signed by the passenger acknowledging that he understands the risks and will not hold you responsible it is impossible in law to cover yourself for negligence. Let's say you fail to carry out some pre-flight check and have an EFATO during which your passenger is injured and cannot work. His lawyer argues that your failure to drain the tank properly was negligent. The Court agrees and bingo your blood chit is worthless.
Not saying one should not do it, just that you should be aware of the potential for problems.