Restricting Calender display to other colleagues

Dear Gurus,

I have a requirement regarding Calendar in web ui.

Requirement is:

Regional Manager wants to see all his sales reps calendar. But sales rep cannot see his manager' calendar and other colleagues calendar. Sales rep has to see only his calendar.

How can I achieve the above requirement. Is there any authorization object to restrict the calender or any customization.

Please reply urgently.

Thanks in advance.

Thanks

Mahesh Babu