Adding Registrants to a Group Booking

I have a user who made a group booking for an event and paid for one registrant, but would now like to add another in their group booking and pay for the extra person, however they cannot as they get the message "you are already registered for this event" . I am not sure if this is because they incorrectly added themselves as the first registrant or not, but how does a group registrant (admin) normally add more registrants after they have already registered someone? Have I missed a config setting to allow editing group bookings?

Further... I notice as an admin I can add more registrants to a group booking in the backend, but not sure how this would manage their payment for the extra registrant(s)?
Also notice that a group booking admin can edit the registrants on the front end but cannot add registrant(s) ?

After much hunting around I changed the config setting "Prevent duplicate registration" to NO, thereby allowing a group registrant admin to add others, which I think should work. However this removes the protection of individuals registering twice by accident (the main benefit of preventing duplicates).
If this is the only current solution Tuan, I suggest that the config option needs to be available for group bookings only, not for individual bookings.