Since version 1.0.0 of Invitation Processor Pro import of attendees has been supported.
Now this seems to be problematic after importing an event into Google Calendar: With attendee data provided for that event the Google Calendar Service starts to send invitations to the attendees on its own.
This is some kind of annoying, since every attendee gets invited again. As a quick solution Invitation Processor Pro 1.1.1 skips adding attendees to the Google Calendar Event.
In future releases I have to find a way how to add attendee data safely, e.g. by appending it to the event description.