To manually add public groups to calendars through Salesforce in order to restrict visibility of your Standard Calendars, Calendar Groups, and Web Calendars, navigate to the Salesforce record of the calendar using its Calendar/Group/Web Calendar ID.
To find this ID, go to the edit page of the calendar.
Navigate to your home tab and append the ID into your URL after salesforce.com/
When you are at the record, click the “Sharing” button.
Click the “Add” button.
Add the group to the record, then save.