2.1.7 [RFC5545] Section, ATTENDEE MUST NOT Be Present When Publishing Calendar Information

The specification states, "This property MUST NOT be specified in an iCalendar object when publishing the calendar information (e.g., NOT in an iCalendar object that specifies the publication of a calendar user's busy time, event, to-do or journal)."

Outlook can export ATTENDEE properties when exporting a calendar in the iCalendar format.