The easiest way to get the format for a meeting request is to create the meeting on Outlook but don't send it, instead save it as an ics file and this will give you the format.
As you can see it is a bit more complex than an simple appointment also you may have to experiment a bit especially with the UID value as Outlook does require this.
BEGIN:VCALENDAR
PRODID:-
VERSION:2.0
METHOD:REQUEST
X-MS-OLK-FORCEINSPECTOROPEN:TRUE
BEGIN:VEVENT
CLASS:PUBLIC
CREATED:20100111T050853Z
DESCRIPTION:\n
DTEND:20100111T060000Z
DTSTAMP:20100111T050853Z
DTSTART:20100111T053000Z
LAST-MODIFIED:20100111T050853Z
LOCATION:Test Location
ORGANIZER;CN="John Smith":mailto:jsmith@work.com
PRIORITY:5
SEQUENCE:0
SUMMARY;LANGUAGE=en-au:Test
TRANSP:OPAQUE
UID:040000008200E00074C5B7101A82E0080000000080FE955ED892CA01000000000000000
0100000001313B641CE540A4799655963034D762B
X-ALT-DESC;FMTTYPE=text/html:<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//E
N">\n<HTML>\n<HEAD>\n<META NAME="Generator" CONTENT="MS Exchange Server ve
rsion 08.00.0681.000">\n<TITLE></TITLE>\n</HEAD>\n<BODY>\n<!-- Converted f
rom text/rtf format -->\n\n<P DIR=LTR><SPAN LANG="en-au"></SPAN></P>\n\n</
BODY>\n</HTML>
X-MICROSOFT-CDO-BUSYSTATUS:BUSY
X-MICROSOFT-CDO-IMPORTANCE:1
X-MICROSOFT-DISALLOW-COUNTER:FALSE
X-MS-OLK-ALLOWEXTERNCHECK:TRUE
X-MS-OLK-AUTOFILLLOCATION:FALSE
X-MS-OLK-CONFTYPE:0
BEGIN:VALARM
TRIGGER:-PT15M
ACTION:DISPLAY
DESCRIPTION:Reminder
END:VALARM
END:VEVENT
END:VCALENDAR