shared calendars don't show up on the recipient side [solved]

[Malte Schmidt-Tychsen]

I found it. The users have to subscribe to the appropriate folders first 
themselves:

http://roundcubemail.sourceforge.net/doc/help/kolab/en_US/_plugins/calendar/sharing.html

"Subscribe to Shared Calendars¶

Calendars shared by others are not showing up right away in the list 
within the calendar view. Switch to Settings > Folders to see all 
resources you can access. There’s a shortcut to this: click Manage 
folders in the options menu behind the gear icon located the footer of 
the calendars list.

In order to make a shared calendar appear in the calendars list, locate 
it in the folder manager and check the Subscribed mark in the list. Only 
subscribed calendars are visible in the calendar view."