Some Outlook users know that they can email appointment details to each other and if you accept the meeting, it is added to your calendar and RSVP’d to the sender. Up to now it’s been limited to sharing between Outlook however Google Calendar http://www.google.com/calendar/ supports appointment sharing in both directions.
In Google Calendar you can create a new event with ‘guests’ and when saving the event you get an option to email the appointment to the guests. When that message appears in Outlook there are buttons to Accept, Decline etc (it depends on which version of Outlook you have). Non-Outlook users see the message text without the special buttons so you can send appointments without worrying about what email client the receivers use. Back in Google Calendar the response will show up in the guest details for the event.
From Outlook, create an appointment and add invitees from the Scheduling tab. If the invitee has a Google Calendar linked to their email address then the proposed appointment will appear in the calendar with a question mark. Hover your mouse over the question mark to see details and pop-up options of Yes, No, Maybe and Delete. In either product, if you change the appointment details there’s an option to advise the invitees/guests. The exchange of appointments works nicely in both directions and it is welcome for both sides.