To share a calendar on an iPhone, follow these steps:

1. Open the Calendar app on your iPhone.
2. Tap the "Calendars" tab at the bottom of the screen to see all your calendars.
3. Find the calendar you want to share and tap the "i" (info) icon next to it.
4. Tap "Add Person" and enter the email address of the person you want to share the calendar with. You can add multiple people this way.
5. Tap "Add" and then tap "Done" to send invitations to the people to join your calendar.
6. The invited people will receive an invitation which they need to accept to access the shared calendar.
7. You can manage sharing settings, including allowing others to edit the calendar or removing someone from the shared list, from the same info screen.

This method works with iCloud calendars and is the native way to share calendars between iPhone users via iCloud.