2.1.6 [RFC5545] Section 3.8.4.3, ORGANIZER MUST be Present on Group Scheduled Calendar Entities

The specification states "This property MUST be specified in an iCalendar object that specifies a group scheduled calendar entity."

Microsoft Exchange Server can export VEVENT components that represent meetings without an ORGANIZER ([MS-OXCICAL] section 2.1.3.1.1.20.16).