Depending on what you want to achieve you can either subscribe to a calendar or share a calendar.
Subscribe to a read-only copy on your device. See Apple's support document Set up multiple calendars on iPhone
Subscribe to a calendar
- Go to Settings > Calendar > Accounts > Add Account > Other.
- Tap Add Subscribed Calendar.
- Enter the URL of the .ics file to subscribe to and any other required server information.
You can also subscribe to an iCalendar (.ics) calendar by tapping a link to the calendar.
Share write-access to edit events:
Share iCloud calendars on iPhone
In the Calendar app, you can share an iCloud calendar with other iCloud users. When you share a calendar, others can see it, and you can let them add or change events. You can also share a read-only version that anyone can view but not change.
- Tap Calendars at the bottom of the screen.
- Tap
next to the iCloud calendar you want to share. - Tap Add Person, then enter a name or email address, or tap
to browse your Contacts. - Tap Add.