Export Calendars from Outlook
If you are unable to locate a shared calendar you had access to in Outlook, you should first confirm it has not been migrated as a calendar resource. Navigate to Google Calendar and in the left side menu, scroll down to Other calendars and click the + icon. Select Browse Resources and search through the (Calendar) section to confirm you do not see the shared calendar listed.
If you are still unable to locate the calendar after following these steps, please follow the guides below to export the events from Outlook. You may then create a new shared calendar in Google Calendar, import the events, and then share the newly created calendar with the other users that need access.