There are many factors including your and their circumstances, who invited who, is this a one time trip or likely to be a recurring venture, how long are you staying, etc. In general, I'd say that the difference in points between what you would have paid without them and what it is costing you now times $10 per point is the gold standard. If you want to give them a break to $8 pp, that is reasonable. You can then divide proportionally. They should also share in the grocery costs. Many would just base it on the yearly maint fees for those points and many other variations you could consider.
When we take family, we don't ask them to pay us for the room, just cover most of the groceries. It's our way of helping them do things they couldn't do on their on. It's also nice to spend some quality time with family we don't get to do very often. We are able to do that without making a big difference to our month to month expenses but I suspect most people aren't in that situation.
What you don't want to do is end up having people feel uncomfortable now or later that you could help but best to handle anything at this time than have to dance later. Also, you don't want to start an ongoing expectation that you can't handle repeatedly.