I remember having a feature like this in a business office using Microsoft Exchange (the server for which often broke down), so I have been waiting for something like this on Gmail. This could simplify a lot of people's lives if well implemented.

Long overdue feature. You could do this with Exchange/Outlook more than a decade ago. I wonder if these meeting invites are compatible with Outlook?

Finally! Now, all that's left is to have something like Loopt with this, so you can check on your friends to see if they are nearby for that serendipitous outing.