Is it possible for a registered user who has made a group booking to edit the booking to add / amend the booking. i.e. add another person to the booking or change some of the custom field options?
You can create a menu item to link to Registration History menu option of Events Booking. Registered users can login, access to that menu item, edit the registration data
Unfortunately, at the moment, we don't have option to allow adding more members yet, only possible to update data of existing members