If you'd like to sync your colleagues' (or family members') google calendars to your iOS device, you must do the following. This article originally appeared in: https://gsuitetips.com/tips/calendar/sync-shared-google-calendars-to-your-ios-device/
Use a CalDAV account to sync all your Google Apps shared calendars on your iPhone or iPad.
- Go to Settings on your iPhone and choose Mail, Contacts, Calendars. Select Add Account and choose Google from the menu.
- Fill in the required details for your Google Apps account and add a useful description eg "Your Company Calendars" - click Next.
- Choose just to sync Calendar* from the list and save the account.
- Once added your main calendar should appear under your iPhone or iPad Calendar app, if you click on the Calendars button in the top left (or bottom middle in ios7) you will see a list of your subscribed calendars - if you chose a good description in the second step above, you 'll be able to see exactly which calendar you have just set up. In all likelihood there will still only be your primary calendar listed.
- To sync your shared calendars, head to this link on your PC, whilst logged in to your Google Apps account: https://www.google.com/calendar/iphoneselect and select which calendars to sync - click Save. You can also visit this link from your iPhone to manage your calendar sync.
- Now refresh your iPhone Calendar list and your shared calendars should appear under the new account you set up.