If a user registers individually for an event, and then wants to book an additional place, how can they do that?
Or if a user makes a group booking for two people and wants to add a third person how can they do that?
I can see that an admin can add or remove members from a group booking but I can't see how a user can edit their own bookings in this way. Are they meant to cancel the original booking and rebook? Or should duplicate bookings be allowed so that additional members can have their own booking?
In this case, that member needs to perform a new registration for new members. And Yes, you will have to set Prevent Duplicate Registration config option to No so that they are allowed to make a new registration
I kind of knew you were going to say that - simpler to handle for the developer, but not for the end user. For free events there would be no payment, and for paid events surely it is just a case of taking a further payment for the new outstanding balance. An admin already has the ability to add or remove members from a group registration (although I have never tried that for a paid event).
From an end user viewpoint the problem with multiple registrations is that you just have a single cancel button and don't know which booking it might cancel. I'm sure you will point me towards the user's registration history to manage their bookings, but that screen isn't needed if a user has a single registration and there are cancel and edit buttons on timeline / category / event details page. And yes I realise that there is an added difficulty if a registration changes from individual to group.