As a PPL, you need to pay for your fair share of the cost of the flight. In other words, if you take two passengers, you must pay at least 1/3 of the cost yourself. There is a limit to the number of people you can cost-share with, too - I think it's 6(?) but I might be wrong on that bit.
If you are renting, there is no flexibility in determining the cost of the flight - add the rental charges to the landing fees, and you must pay at least 1/3 of that (again, assuming two passenger). But I can't see any problem if your passengers also decide to buy you lunch, and pay for the petrol to and from the airport, just as a way of saying thank you.
Hope that helps.
FFF
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