There are four different ways to add new calendars.

In your calendar, click the "+" next to "My calendars" and select "Add new calendar". Type a name for your new calendar and tick the box next to "Add as public calendar" if you want to make it available to others in your company. 

To Subscribe to a calendar someone has shared with you, click "Subscribe to shared calendar" and the calendars that have been shared with you appear. Click the button next to the calendar you'd like to display and then click "Save".

To use an iCal link, click "From URL" and enter the iCal URL. Then click "Subscribe". Your calendar will appear under My calendars.

To import a calendar from a file click "Import" and then click "Upload file", enter the new imported calendar name and then click "Import".