Hi,

the enclosed patch adds two features to gnus-icalendar:

- the article buffer now shows a new field 'Attendance' with values of
  either 'Required' or 'Optional' - good to know which meetings are safe
  to ignore

- appointments exported to org outline now contain additional properties
  PARTICIPATION_REQUIRED (t when required), REQ_PARTICIPANTS (list of
  required attendees) and OPT_PARTICIPANTS (list of optional attendees)