I have used "Event Booking" for free event with paid additional services. I have opened both - individual and group registrations. But group registration has not summarized Custom Fee Field values. Pretty quickly (in 3-4 days) I got solution from developer. This Event Booking solution has some disadvantages more (for example lacking "max number" option in Custom Fee field), but basically this now works as needed. I think, it's a good solution for Event Booking for Joomla 1.6 at the moment.