The calendars can be shared within selected users 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.
Users who are into Administrators group can edit shared calendars created by other users. The changes made by administrator affect source calendar. To edit user's calendar it should be shared by author and administrator's group added to share permissions.
Please notice that before adding shared calendar the user must create his/her own calendar. The settings for this calendar don't matter.
Press Edit button and select Share in dialog window:
Select groups from dropdown list and press 'Add' then Save settings. Selected goup(s) then will see this calendar in Shared calendars list and can add it to his/her own list.
First press 'Add Calendar' button/ 'Add shared'. In 'Shared Calendars' tab choose available calendar you need and press 'Add':
This calendar now appears in 'Favorites' tab and in dropdown list of available calendars: