Sync a Shared Google Calendar with macOS, OS X, iOS (iPhone, iPad), or CalDav
2018 Update: Sadly, this is still the correct way to get syncing working.
2019 Update: Still true.
2020 Update: Sigh.
2021 Update: Yup, still relevant.
2022 Update: Another year, still relevant.
2023 Update: Still works
2024 Update: How is it possible Google still hasn't fixed this?
Someone shared a Google Calendar with me recently, and it showed up at calendar.google.com just fine. However, I could not see the shared events on my Mac (using Calendar) or on my iPhone or iPad (using the Calendar app).
To fix this, I had to visit https://www.google.com/calendar/syncselect directly, and check the shared calendar, then save the settings.