John, if you are trying to share a local/personal calendar with other users, click on the calendar name in the left nav pane. It will open a modal on which the second tab has sharing information. You can configure other others and/or groups with desired permissions. If the calendar is a domain calendar, in domain settings go to Shared Resources and create/edit a calendar there. Again, on the modal you would config the sharing permissions.
For users that have shared calendar available for them, can click on the bottom left icon (see below) and select Map Calendars. This modal lists all calendars that have not already been attached.