Go to icloud.com on a computer. Open the Calendar app. Next to each of your calendars is a little wi-fi looking icon. Click it and you will get sharing options. You can share the calendar either private (editable) or public, but only with other iCloud users.
The other iCloud user will get an email with a link. They accept/add the calendar by tapping on the link.
More:
Just check, and it's now possible to share the calendars directly from the iPad, if you have iOS 6. Tap on the Calendar button at the top left, choose Edit from the top, tap the arrow to the right of the calendar you want to share, and Add Person. The rest should be the same.
I haven't tested this, so if it works, or not, please let me know.