PD,
cannot say I have ever experienced having extras along for the ride who were not necessarily carers for the sick child / person. The most I was ever asked to carry was three including a cancer sufferer with not long to live, their spouse and child.
I think Angel flight would vet pretty carefully those they are putting up for the flights for the very reasons you mentioned.
Back to the original topic though, I see no issue with offering an earth angel a flight given that they also sacrifice their time to look after the patient but only get a boring as bat s**t drive for their trouble.