The calendars can be shared within selected user groups by calendar creator. Shared calendars can be edited by their's creators and administrators but issues in calendars can be edited by users with correspondent access to Issues and/or Projects in Permission schemes. (warning) The only creator can remove the calendar. The changes made by an administrator affect the source calendar. To edit a user's calendar it should be shared by author and administrator's group added to share permissions.

Share calendar

Press the Edit button and select Share in the dialog window:

Select groups from the dropdown list and press 'Add' then Save settings. Selected group(s) then will see this calendar in the Shared calendars list and can add it to his/her own list. Also, you can grant permission to Edit the calendar. 

The 'Edit permission' allows configuring a calendar for a certain group(s). 

Add shared calendar

First press 'Add Calendar' button/ 'Add shared'. In the 'Shared Calendars' tab choose an available calendar you need and press 'Add':

This calendar now appears in the 'Favorites' tab and in the dropdown list of available calendars.