Public Calendars can be managed by multiple admins. You can invite people to become admins via invitation URL links.
STEP１：Open the manage menu from your public calendar.
STEP 2 : Select Manage Members
STEP 3：Tap Members list shown in the image.
STEP 4：Obtain the invitation link by tapping the invite button.
STEP 5 : Share the URL via LINE, Facebook Messenger, Email, Twitter.