The issue came if you put a group (while you are writing it it autocomplete as it should) in the contact list, when you click "apply" a dialog appears telling that "it's not a correct address... would you like to add"? And if you click yest, kontact crashes
this report doesn't read like a calendar bug. it looks more like a contact bug. there's also no backtrace and it refers to an old version. therefore I will close this. if the bug still happens with a newer version (like version 6.4) please provide a crash backtrace
oh, I missed the "participant" dialog so it is probably a KOrganizer bug. even so, there isn't much I can do without a backtrace for a modern version.