Google Calendar - Responding To A Meeting Invitation

There are a few ways to respond to a meeting invitation in Google Calendar:

Email Notification
An email invitation sent to you by the person setting up the meeting contains controls to respond to a meeting.

The full message:

A zoomed view of the response area:

On the Calendar - Overview Window
You can also respond to an invitation from the overview view in your calendar via the pop-up overview window. This allows you to respond without opening the detail view of the meeting.

On the Calendar - Detail View Window
If you click into the detail view of the meeting, there is a response area that indicates whether you have responded and/or what your intention is regarding your attendance at the meeting.