I had a question from an Outlook.com user who shares the calendars with other familiar members. When the accounts were on the old server, the calendars were editable in Outlook but aren't on the new server.
The family shares Outlook.com calendar (new server) with each other but in Outlook 2016 I can only view the shared "Outlook Mail" calendars. I cannot add new events to them. They are all read only.
Correct, at this time, the shared calendars are read-only in Outlook desktop.
Outlook.com supports sharing calendars with editing permissions, however, the recipient needs a Microsoft Account for their address. If the Outlook.com account assicated with the Microsoft account is added to their Outlook profile, they will see the calendar in Outlook (under Other Calendars group). At this this time users cannot add new events or edit existing events to the calendar in Outlook.
Recipients will need to add events to the calendar using Outlook on the web. Shared Calendars also editable in in Mac (because Mac uses web services to sync).