would only charge my friends up to 100% of the operating costs of the aircraft for the time (rent, fuel, landing).
You can't do that - at most, your passengers can contribute an equal share of the cost as you - e.g. if you have 2 passengers they can contribute at most 2/3 of the cost. You should get the passengers to settle the insurance cover separately with the club.
Also, you cannot accept any payment or expenses relating to the flight, other than the cost sharing above. Nor can you advertise the "service".
Otherwise, there is no problem with taking your friend's parents flying.
SD