You can buy on their behalf, just assign each ticket to someone with enough points. They need to already be in your Friends & Family or add them in as part of the buying process.
Works fine for me, and I have been one of the biggest critics of it (and still am in some regards).
Maybe it's because I've bought a fair few times that I know that this...
You have selected an invalid number of tickets either for this ticket owner (McKay, Ben), at this price type, or for this...