Or an alternative stance to that stance is to spell out whether an invitation RSVP is needed. If not knowing who the hell is coming or not in advance is immater

Or an alternative stance to that stance is to spell out whether an invitation RSVP is needed. If not knowing who the hell is coming or not in advance is immaterial to the event, then just make it an open invitation ("Hey we are hosting blah blah at blah on blah. Hope you can be there. Just show up!"). But if you're being asked for a reply by a certain date and you miss it, you should reasonably expect not to be allowed to the event. And, if you do show up anyways, you might get in and you might end up being accommodated by the hosts/organizers regardless. But that wouldn't make you any less of an ass.