With Google Calendar's sharing settings and an external application you can set granular controls on how your calendar is shared inside and outside your organization.
Collaboration in Google Apps is not just restricted to Google Docs and Google Sites. You can also add collaborative features to your Google Calendar as well.
Collaborate with individuals
With the Google Calendar sharing settings you can share the details of your calendar with your entire organization. You can also restrict sharing in the organization and specify individuals who have access to view your calendar (your manager) and individuals who have access to make changes to your calendar (your assistant, your significant other, etc.).
Share your calendar with your company
When you share your "free/busy" availability, the people to whom you share your calendar will be able to see the blocks of time when you are busy, and when you are free, but not the details of your events. Sharing your calendar in this way allows users to book meets with you at times when you are available, instead of having to take a guess at your availability.
Allow users to make changes to your events
When you give users access to "see all event details" they can see the who, what, when, and where of your events, but they cannot make changes to those events. This type of access is helpful for your manager, team, or direct reports who need to be able to locate you or help you plan your projects.
You can always indicate that certain events are "private", like a medical appointment. When you mark an event as "private", even those people who you share your event details with will not see the who, what, when, or where of these "private" events.
Subscribe to public calendars
Google Calendar also allows you to create secondary calendars that can be shared with teams or the entire organization. The Human Resources department may want to create a calendar that displays the company holidays. By making this a "public" calendar, employees can quickly subscribe to the calendar so that it will display overlaid on top of their own calendar with the click of a button.
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Open appointment slots
The most recent collaboration addition to Google Calendar is the Appointment Slot feature. This lets users designate certain blocks of time that are open to others to select a time and book an appointment. Appointment slots are helpful for team office hours, performance reviews, supervisory meetings or student conferences.
The calendar owner just selects a range of time when they are available for an appointment. They also select whether the appointments should be broken into increments of a certain time period. A special URL for the appointment booking page is generated. This URL can be shared with users within the organization or outside the organization so that they can book appointment times.
When a coworker books the appointment they can enter the "where" and "description" details. The event will be placed on the requester's calendar and the appointment slot will appear as booked to the person offering the appointment slots.
This method is helpful for use within an organization. The feature can only be used by those people outside the organization if the user also has a Google Calendar account. They must sign in to their Google Calendar account in order to view the appointment booking page URL. The appointment slot feature is a step in the right direction. However, it is not a realistic appointment booking system for businesses who serve clients that do not necessarily use Google Calendar. This would include attorneys, salons, home repair services, etc.
Extending appointments with Tungle.Me
Tungle.Me is a scheduling application that syncs with Google Calendars and allows users inside and outside the organization to quickly and easily book meetings.
When you sign up for Tungle you create a profile page that can be shared with prospective clients or coworkers. Those individuals can see your free/busy schedule, but not your event details. You indicate the times when you are available for meetings and the people who want to meet with you can propose several time slots (you choose the number) when they can meet as well. You receive an email, confirm the meeting time, and the meeting time is booked and synced to your Google Calendar.
Using a service like Tungle prevents double bookings and saves time. You no longer have to email back and forth about potential availability or spend time trying to figure out how to get your calendars to sync so you can share availability.
Tungle also offers additional customizations that Google Calendar Appointment Slots does not have. With Tungle you customize the hours when you are available for appointments, how far in advance the appointment must be booked, and how many potential time slots a user must suggest. These controls prevent you from missing meetings, overbooking or having to decline a meeting.
The client who uses Tungle to book a meeting with you does not need to register with the service and does not need to use Google Calendar.
Admin Control of Tungle
Tungle is in the Google Apps Marketplace which means that a Google Apps Administrator can enable or disable Tungle for the entire domain. The Admin can also select individual users who can use Tungle, while disabling it for other users.