ok, I made it clear and it's only for the third guest:
Dear Sir,
You don't need to pay 500 THB each time but each time you can bring only 1 guest with you.
You can bring guests many as you like per day or per night depend on you but each time only 1 guest for 1 or 2 hour and change to another one there no problem.
But whenever you bring 2 guests in the same time the 2nd guest will be charge 500 THB.
So if you don't want to pay 500 THB just bring 1 guest each time.
And the important thing is do not forget to inform you guest to leave their ID card or passport at the reception desk for your own security.
Best regards,