The web app would need to create the appt transaction in the correct format, place in the corret folder on the GroupWise server, and deal with the cleanup of messages after being read by GW.
I looked on the Novell Support site and couldn't find the SDK documentation I had used years ago to convert from another calendaring system to GW. I think they call it something other than SDK (System Developer Kit) now. If you can locate it's equivalent on their site, it should contain the reference material that describes the message format, the appropriate GW directory to place messages in so GW will process them.
Sorry I can't find the link you need. Maybe some other, brighter help will come along.....
dennis