If I got invited to a friends/family members ceremony I would go - there'd be no question about it. I think if you came up with some clever wording then it owuld be fine, so long as people know in advance then they would be fine with it. Now if you did it the way a couple I know did it, where there was no warning the guests had to pay for their own very exxie dinner and the first they knew about it was when someone came around asking them for the money, then that would be rude ROFL.