Howto: Set the organizer of a meeting on the calendar of an attendee using EWS.

I would like to say that this is something which is easily possible - however, I cannot. Setting the organizer directly in code is not supported. Historically, having the organizer over-writable has caused some pretty bad situations for customers. When a meeting is created, the organizer property is automatically hard-stamped and cannot be specified during meeting creation.   Further, the organizer field is read-only and documented as such.  Direct log-in, Impersonation and delegate access will not change this behavior.  At the time you cannot directly overwrite it with possible exception of creating the meeting's using MIME  - which is not supported and complicated. 

Appointment.Organizer Property  

If you decide to create the meeting using MIME, have the MIME/VCALENDAR content use METHOD:PUBLISH instead of REQUEST.  The problem with this is that there are other properties which need to be set. There is no documentation or recommendations on what all would need to be set or if methodology might work in all cases.  Further, you might get it to work reliably for now, however it may not work as expected in future updates to Exchange.

Note this for delegate access with EWS:

Creating and Sending Meeting Messages on Behalf of a Principal (Exchange Web Services)

The Organizer and From properties of the meeting invitation are set with the principal's mailbox. The Sender property is set with the delegate's mailbox. The relationship between delegate and principal are expressed in Outlook Web Access 2007 as "[Sender property] on behalf of [From property]."