Using New Outlook on Windows for personal email, calendar, and contact management
Dear Jeremy Millar
Thanks for choosing Microsoft Community.
Based on your description, I know the scenario you met. For this, to narrow down the situation, please be kindly to let this user log in his/her account in outlook on the web and then try to add shared calendar on the web side. And check if he/she can do this on the web side. Here is the reference: Share calendars in Outlook on the web - Microsoft Support.
Meanwhile, I need to confirm with you only user meet this scenario? If so, please also be kindly to let this user create a new windows profile in his/her laptop. And then use this new windows profile to open outlook then check the result. Here is the reference: Manage user accounts in Windows - Microsoft Support.
If there is any update, please feel free to update me. I will continue to check for you further. Your patience and cooperation will be highly appreciated.
Thanks for your precious time. Have a nice day.
Sincerely,
Katherine