You absolutely categorically MUST write up an agreement that covers all the issues associated with hiring the aircraft, such as insurance (who pays) public liability, passenger liability, mechanical failures, fuel, air services charges, landing fees, call-out fees, who does what in the event of an incident or mechanical problem, how the aircraft will be run and tied down or otherwise managed when not flying....
Check out the hire agreements that aeroclubs use as an example / starting point.
Do yourself and the owners a huge favour and get an aviation lawyer to write up the agreement. It's all very well saying 'she'll be right mate' but when the poo hits the fan a small misunderstanding can become a massive financial and legal burden very quickly.