You need the “Modify Calendar” permission to add a calendar to a group. See, Assigning Calendar Options & Associating Calendars with a User Group.
2. Click Add Calendar to display the following panel.
Tip:
4. Check the Is Calendar Group checkbox. The Associated Calendars pop-up displays.
5. Click the Calendar Name drop-down button to display a list of Individual Calendars and select a calendar.
8. Click next to the Calendar Name to insert and save the calendar.
10. To cancel the Add Calendar mode click the Cancel Add Calendar link.