Note: Sign in to Outlook on the web using any Microsoft account, including email accounts ending in @outlook.com, @live.com, @hotmail.com or @msn.com — and Office 365 work or student accounts.
Outlook accounts were recently upgraded, and shared calendars were temporarily disabled during the upgrade. Your calendars will be fully re-enabled once everyone who shares your calendar is on the new Outlook.com.
Even after calendars are fully re-enabled, you cannot edit calendars shared with you if you're using Outlook on Windows. Also, please make sure that you recreated your account in Outlook on Windows after you were upgraded. For more information, see Reconnect Outlook 2016 or Outlook 2013 to Outlook.com to fix issues after Outlook.com upgrade.
If you add items to a calendar in the iOS Calendar app but they don't appear anywhere else, confirm that you have edit permissions to that calendar. The iOS Calendar app doesn't prevent you from editing or adding items to calendars where you have read-only access.