We just got back from a trip - 3 of us had APs and one of us (a non-family member child) had a 5 day park hopper ticket. Prior to our trip
MDE gave me a message that I could only add
Genie+ for members of my party with the same ticket type as me, and that others would have to purchase Genie+ themselves through their own MDE account. I chatted with a cast member before our trip about that message because I thought I might want to add Genie+ for certain days of our trip and didn't want to run into any trouble with the child's park hopper ticket. He doesn't have and wouldn't have his own MDE account. I made that clear to the cast member. The cast member on chat assured me that I would actually be able to purchase Genie+ for everyone the day of. That wasn't true. One morning I was toying with getting Genie+ for us all and it would allow me to purchase for those of us with APs, but not the park hopper. I wasn't about to make an MDE for him (he's not my kid) and I didn't feel like I had an effective way to deal with it at 6:30 am, so we just skipped it. We had also had bad luck with Guest Relations and a prior issue trying to get an ILL for my husband (I could purchase it for 3 of us, but he was "ineligible because it was too early for park hopping", which made no sense since his park reservation was identical to ours).
All that is to say that I didn't see anything that led me to believe that one person can buy Genie+ for people with a different ticket type, and for whom they don't control the MDE account.