Let's test that exporting a event with attendees works just
fine. First step is creating a event:

  >>> from DateTime import DateTime
  >>> _ = self.portal.invokeFactory('Event', 'event1',
  ...      attendees=('Foo <foo@bar.com>', 'Bar <bar@baz.com>'),
  ...      location='Vienna',
  ...      title='Plone Conference 2004',
  ...      startDate=DateTime('2004-09-20 01:00:00 GMT+0'),
  ...      endDate=DateTime('2004-09-24 04:00:00 GMT+0'))

Now let's export it:

  >>> print self.cal.exportCalendar(events=[self.portal.event1])
  BEGIN:VCALENDAR
  PRODID:-//Runyaga.org/NONSGML Calendaring Tool//EN
  VERSION:1.0
  BEGIN:VEVENT
  UID:1445300814-1921707519-RID
  SUMMARY:Plone Conference 2004
  DESCRIPTION:
  LOCATION:Vienna
  ATTENDEE;CN="Foo":mailto:foo@bar.com
  ATTENDEE;CN="Bar":mailto:bar@baz.com
  DTSTART:20040920T010000
  DTEND:20040924T040000
  DTSTAMP:...
  STATUS:TENTATIVE
  END:VEVENT
  END:VCALENDAR