How to Set Up Calendar Permissions for Multiple Users in Microsoft Graph?

Since you're using M365 accounts, consider managing these permissions directly through the MS 365 admin portal or the Exchange Online module for a more straightforward process. Leveraging Microsoft Graph can be tricky at first, especially with permission handling.

Your error indicates that `$primaryCalendar.id` isn't a string. This can happen if there are multiple calendars returned for a user. Check the contents of `$primaryCalendar` to see if it’s returning more than one calendar and ensure you're pointing to the correct one.

I noticed you set `$primaryCalendar` as an array first and then immediately overwrite it with a single calendar. This could lead to confusion. Since you're targeting one user's primary calendar, you should just assign it directly and avoid the array initialization. Also, make sure that `$primaryCalendar.id` is indeed a single string and not multiple values.

It looks like you're trying to fetch the user calendar permission but the CalendarId might not be formatted correctly as a string. Try using `$existingPerm = Get-MgUserCalendarPermission -UserId $targetUser -CalendarId "$($primaryCalendar.id)"` to force it to convert into a string. You might also want to print out what `$primaryCalendar.id` returns just to be sure it's what you expect.