Create a new calendar. Click in the menu "File" on "New" followed by "Calendar".
Choose the calendar-typ "CalDav".
Enter all required information:
Name: the name of your calendar
URL: caldav://groupware.stepping-stone.ch
Tick "Use secure connection"
For "Username" and "User email"add your email address.
Followed by clicking on "Browse server for a calendar" to browse for the correct calendar.
The server now requires your password. Enter your password and confirm with "OK".
Now you can see, which calendars are available. Select the desired calendar (your calendar or one of the calendars of your colleagues) and confirm with "OK".