This is on my mind right now!
I find this thread very interesting as we have recently purchased a beachfront villa in Mexico. Suddenly our beachfront home has become the hottest ticket in town with all our friends! So we found that we had to come up with some ground rules.
My husband and I agreed that if our friends join us while we are at the villa, then we consider them our guests and would never ask them to pay anything. We have 3 bedrooms and 3-1/2 baths so we have plenty of room! If they choose to take us out to dinner or buy us a "host" gift, then we'll accept with pleasure, but there is absolutely no expectation on our part. On the other hand, if our friends would like to borrow the house when we are not "in residence", then we have decided that we will charge them a nominal fee of about $50 per night, intended to cover the maid service after they leave, costs of getting someone to let them in, close up after them, contribute towards our HOA fees, other costs, etc. Plus, we will be clear with them that this is not like staying in a hotel, so they will be expected to clean up after themselves, run a dishwasher load and laundry load when they leave, etc.
Most importantly, we decided that we will be upfront and clear on our "policy" with our friends - I am a firm believer that clear upfront communication avoids misunderstandings, and I still want to be friends with these people after all is said and done!
Now, having said all this, we won't be applying this rule to immediate family -- there's no way that I would ask my parents or in-laws to pay a penny. Ditto for my brother and his family. Cousins, we'd probably charge them.