I don't know, it all depends on the situation. If I invite them, they pay nothing and I expect nothing. But if they say hey can we come, and I had no intentions of getting a larger unit, well then they would definately help pay the difference.
We have a 2 bedroom for New Years, we invited my parents, they said no, so I offered my inlaws the space for 2 nights as a Christmas present, 2 nights over New Years, with the kids, plus birthday celebration for my son at Hoop Dee Doo. Now my parents decided, gee maybe we do want to go. We could squeese them in, that would be 8 in a 2 bedroom, not my idea of vacation. so I said your welcome to join us, but I am out of points, try to get a room, or I will try with my
DVC discount. Sounds harsh, but they should have taken me up the first time!!! Plus why should we be crammed!