Hi @Joan Burg ,
As I know, for New Outlook for Mac, the Shared calendars feature is only available in Beta Channel version yet.
So, if you would like to using the Shared calendars feature in New Outlook for Mac, please try to upgrade your Outlook to Beta Channel version, the steps mentioned in this document should be able to help: Early access with Office Insider.
After upgrading Outlook to Beta Channel version, you could open your shared calendars via File > Open > Other user's folder.
Hope this can be helpful.
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