When we went to stay with my brother a month ago we pitched in.

We payed for all our own food that we needed plus paid for several other lots of groceries.
We paid for petrol.
We paid for everyone to go to the zoo.
We also got them a gift for having us to stay - even though this is my brother and his family. I got them something that i knew they wanted. Spent a fair bit but that is what we wanted to do.
We helped with house work and did our washing plus theirs. Etc etc etc.

It wouldnt matter who we were staying with - we would still pay our way.
I just say "we are paying for that" and dont let them have a say in it.

I dont know how you would ask - we had my SIL's father and his GF stay with us quite a few years ago and SIL promised that her Father and his GF would give us money. Oh man - they stayed for two weeks and didnt give us a cent ! We were going out of our minds - we had to do everything.They are just a really annoying couple. We said they could stay to do SIL a favour cause they didnt have room at their place. Didnt know how to ask them for money ! It was awful. At the time we werent in the financial position to do that either. We REALLY struggled !

I hope your guests just pay for some groceries and you dont have to worry !
Good luck - sorry i havent been able to help.
xxoo