Email. I don't tend to organize things with dozens of people on the list (for that I might be convinced that random mail threads might become cumbersome - I'm not convinced that FB is a solution!). If I want to go to a concert and discuss that with <= ten friends, mails are fine. That reply-all argument doesn't come up. If someone has to decline it's trivial for any participant to remove that address. Or he explicitly asks to be removed. Or - and that's probably the general case, that person doesn't care about a couple of emails and marks the thread as read. Done.

I don't claim that you make this issues up or that these things cannot be a nuisance for some people. I .. just don't have these problems, mails work just fine in all cases here.

I agree that a calendar is missing. I tend to use various things for that and don't have a good solution for this so far. Most of the time it's probably 'no calendar' and only a poll of sorts (think doodle or something similar).

The outcome was that some people simply aren't reliable to follow up if they're fine with the event terms or not, no matter what medium. It's not a tech problem, it's a people problem and everyone's responsible.