For those that are still not getting it....
THE. OP. IS. NOT. TALKING. ABOUT. A. PARTY. 

We largely agree that inviting people to a party at your home or a venue means you cover the basic food and drink costs, and perhaps people bring a bottle or two to contribute.
A event / activity / sit down meal out / trip to the pub is a different entity. I would never, ever expect anyone to pay for me on any of the above nor would I be at all offended to be asked (not summoned as some of you seem to think) to join in. I'd be delighted to, 9 times out of 10.
In my world people have different tastes, different budgets, different abilities, different responsibilities. Sometimes we'll go dutch on a bill, sometimes someone is feeling flush and will treat or pay a bigger share, sometimes someone is struggling so we'll chip in and cover them, rather than them not come. Sometimes we adapt a plan to suit someone with kids, or because someone gets seasick, or whatever. Swings and roundabouts, it all evens out in the end. We always do our best to accommodate everyone because we like each other, care about each other and actually want to spend time together. Some respondents here seem to have very funny ideas about what friendship is.