Calendar sharing can be found in the calendar settings.
Properties > Permissions
There, first add the desired user via the address book and then edit the permission level.
You can also share the calendar with other people under "share calender" via e-mail.
When sending the email here, note that calendars are only shared with read permissions.
To enable editing of the calendar, set a check mark for extended permissions when sending the e-mail or assign permissions in the calendar settings.