julia1983 : Maybe it’s cultural, but where I’m from (U.S. urban coastal city), if you’re having a formal party, you don’t expect your invited guests to help you pay for it. It’s not just impolite, it undercuts the very notions of hospitality built into the idea of a formal event. For this same reason, a cash bar would be shocking: if you’re inviting me to a party, you’re implicitly offering me food, drink, and entertainment; to expect me to provide my own, even by paying at a bar you’ve provided, seems counterintuitive and contradictory to the very notion of a “party.”
Now, there are gatherings in which it’s okay to ask guests to bring something, but these events are usually collaboratively organized. So, think a super casual house party, the kind usually thrown in your early to mid twenties; or dinner parties, where it’s good manners as a guest to bring a bottle of wine (but not usually a dish. And if a guest offered to help pay for the ingredients used in the meal, I’d be mortified), and, closest of all, the potluck, where you ask everyone to bring a dish so you can all eat together — but that is a very very casual event, often associated with warm-season holidays, outdoor barbeques, gatherings in backyards and public parks, and so on.
Edit: The longer I think about it, the more I feel that the “collaborative” element is key to deciding when it’s okay to ask guests to contribute vs. when it’s not. There is nothing collaborative about a wedding reception; it is an event you throw to mark a huge occasion in your own life. Your wedding would happen, presumably, even if the guests weren’t present, so it’s the opposite of collaborative. Hence, no, you can’t ask guests to contribute.
A dinner party — well, you’d have dinner if the guests didn’t come, but their presence is important for making it a dinner party, so of course they can chip in by bringing a hostess gift of a bottle of wine, or maybe their famous cheesecake, the one everyone loves but can’t manage to replicate.
A potluck – wouldn’t happen without everyone bringing something. Wouldn’t even BE a potluck if people brought nothing. Completely collaborative.