You can set up personal private calendars for individuals and give an assitant the ability to update a manager's calendar.

1. Login as administrator and go to the Admin Menu, click on Users and add your users. 
2. Go to the Admin Menu, click on Categories and enter one category for each personal calendar (name the category such as "John Doe Calendar").  Select option C, "Allow only selected users to view this category..".  Then select the user for this category in the list of users on the right side of the screen.
3. Go back to Admin Menu, click on Users, edit each user and assign them as a manager of their category.  Also edit any assistants and assign them as managers of any category they will be managing.