When NetCommunity event registrations are processed, it assigns registration fees to each person, regardless of whether they are a participant or a guest.  This is an issue if you add a guest from NetCommunity to a grouped event, because grouped events do not look at registration fees on guest records.  To get around this, guests from NetCommunity must be added to the grouped event as if they are they are primary participants in order for all registration fees to be accounted for.  See How to register a participant for one or more events in a grouped event for more information.