You’ve got it 🙂
You could book your own ticket separately with cash then use the airpoint dollars for the other members. It’s sort of the same thing. Then call Air NZ to ask them to link the bookings just in case they need to juggle passengers around – RARE you’d lose your seat but it happens. Try social media (save some phone time).
I don’t travel with my partner that often but when we do I typically use the airpoint dollars for his ticket but we still split the cost of the paid ticket which is cheaper. In a roundabout way it’s a points and cash option and we’re basically getting there for half price.
I just had a quick look at the flexipay option with airpoints for domestic fares and it has some flexipay cost in there as well, and if you have to pay the rest by credit card you also get charged for the credit card fee.
You are far better off booking full fares using airpoint dollars and paying outright cash for the other.