Never heard of guests being asked to pay their share for the Christmas dinner tbh
I've never been asked to pay and while I never hosted an xmas dinner before, I don't think I could ask people to pay if I'm inviting them over. (That being said, I'm not married and I don't have in laws or relatives that expect/hope/demand to be guests and I am free to choose whether I want to entertain or not. Not saying that married people can't choose
but as far as I can see, the whole Christmas dinner thing can be more tedious if you have a bunch of in-laws to consider.)
I personally wouldn't be offended if someone asked me to chip in but I'd rather do that in the form of bringing a side dish, drinks, dessert or what have you.