I actually thought of Google calendar as soon as I read your post, too, but didn't reply, since it didn't do exactly what you wanted.
I have used Google calendar both professionally and personally.
At the office, I schedule my patients on Google calendar, and I access it usually on my iPad, but also various Windows PCs online if I'm near a computer, a Mac Mini via iCal in my office for my helper to view, and my android phone on the road. I gave 'view only' permission to a company in Arizona so they can make telephone appointment reminder calls for me.
At home, I have a separate Google calendar which my wife and I share and she accesses it via her android phone and her MacBook Air, and I view it through my iPad and android phone, and online on whatever computer I'm near. My iPad and android phone have two separate calendars in different colors - one for work, one for personal. For the first time ever, I can actually see the things my wife plans for us when she enters something in our calendar on her phone.
And it's all free!!
Probably too much info, but the point is that I'll bet you can set up a calendar which will be accessible and updated in near real time (typically seconds), to several people through many different vehicles and with many different permissions. There's some kind of 'invite' function which I never deal with, but maybe that could be used to email people.
Good luck!