You need to do these steps.
1. Create the Calendar App on the SharePoint site you want the calendar.
2. Go to that Calendar, calendar tab, Sync to Outlook.
3. Now change your view on your shared calendar to list view, make sure it's only that calendars item's showing, sometimes I have to do this twice cause it likes to grab your calendar items in that view too.
4. Ctrl+A to seleect all those items.
5. Drag those items to the SharePoint calendar that is now synced in your calendar list.
This will import all the items into the SharePoint calendar, which is now sync'ed. Remove / set your shared calendar to read only to prevent new items going there instead of the new SharePoint calendar.
Instruct others to go to the calendar and sync it with Outlook "If preferred" or they can just access / add items in the SHarePoint pages.
