Hi Meg,
Select your own calendar instead of Shared Calendar to check if Overlay option populate.
It may be your profile side issue. Create a new Outlook profile then test- https://support.office.com/en-us/article/Manage-profiles-or-identities-in-Outlook-for-Mac-fed2a955-74df-4a24-bef6-78a426958c4c?ui=en-US&rs=en-US&ad=US
Have a good day!