Ncal2gcal uses the google calendar api (via the Gcal4ruby gem). In the documentation of the api you find the following description regarding participants:
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Note that if you use the Data API to add an event to your calendar that involves multiple participants, the event appears on the other people's calendars just as it would if you added it by hand using the Calendar GUI.
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To avoid multiple invitations I plan to sync the participants as part of the description.