While you can easily drag a mail message to the calendar to open a new
appointment window (which will include your mail message as text in the
notes pane of the appointment), there's no way I know to have Outlook fill
in the message recipients as attendees.

An addendum to my prior post.  There is a fairly easy way to add the
attendees, if they're all in your Contacts, the GAL, or their explicit
internet address is given.  Drag the message to the Calendar, creating a new
appointment.  Leave the new appointment window on the screen.  Go back to
the original message, right-click on the recipient list and choose "Select
All".  RIght-click again and choose Copy.  Now in the new appointment
window, click the Scheduling tab, then click "Add Others" at the bottom and
choose "Add from Address Book".  Right-click the Required (or Optional, as
you wish) field and choose Paste.  Click OK.  That should add the recipients
as attendees.  It did for me.