How do I politely tell my guests about Hotel arrangements and that we will be accepting payments?

One of the hotels I looked at has a similar policy but all that is required is that any rooms not taken by guests are then paid for by ourselves. We decided against it. But the hotel is the one that would be taking the bookings in that instance and the guests would not know one way or another. Speak to them to see if they will deal direct with the bookings, it makes more sense as they need to know numbers etc with regards to breakfast, and ask them to then reduce the bill by that amount or even to refund yourselves once the event is over. There really is no polite way to do this and no way of guaranteeing that people won't try to take a free ride if they find out.