Go to Company > Company Profile > Default Calendar Settings.
By default, all entries on the calendar are set to Self.
You can choose from:
Self: Employees can only see the events that apply to themselves.
Immediate Reporting Employees: Only the direct reporting managers of the employee can see events that apply to them.
All Reporting Employees: A manager from the top of the reporting chain can have a bird's eye view of all of the employees reporting to them and their subordinates. The direct reporting manager can also view the their subordinates' events.
Department Employees: All employees within the same Department can see each others' events.
Work Location Employees: All employees within the same Work Location can see each others' events.
All Employees: All employees in the company can see all events.
These settings will apply to all employees by default.
Changing the calendar settings for particular employees
To customise the calendar settings for particular employees (e.g. admins), you will need to go to the individual employee's profile.
Go to My Employees > choose the relevant employee > Employment > Calendar Settings. Make the relevant changes there.