OK, I've sorted out the calendar thing, but it is shame that I can create new calendars only on the SOGo web-interface.
Jabber is still bugging me. I've tried a few things and it turned out, the users can communicate, but they still can't see each other in the user list. Why?
Adding user with:
Username: user.name
Host: hostname
It logs in, I can add users and chat with them, even if they are shown as offline. But why do I have to add them one by one manually and why do they show offline?