I recently picked up an iPhone 5S running iOS 7. My wife and I use Google Calendar to share each other’s schedule. She was able to see my calendar on her iPhone 5 (iOS 6) but it disappeared when she upgraded to iOS 7. Here’s how you can get it back.
All you have to do is open http://www.google.com/calendar/syncselect in your device’s browser. It may ask you to log in to your Google account. Afterwards, the page should list your calendars as well as your shared calendars. Just check off the calendars you want displayed. It should automatically display during the next sync.