When creating an event: Toggle the "optional" status of an invited guest by pressing Tab to get to the guest list, pressing the up and down arrow keys to move the focus to the appropriate guest, and pressing the spacebar to change the status.
To edit an existing event, go to the Agenda view, tab to the event, press Enter, then tab to the Edit link.
In the list of Other Calendars, toggle the ability to view a friend's calendar by using the Up or Down arrow key to move focus to the friend's name and then pressing the spacebar.
