On our first park day next Monday 4 of my 5 family members will be arriving at the park at rope drop and my son will fly in and join us later that afternoon. I plan to purchase genie+ at rope drop. Will I be able to do that for all of us or only the 4 of us currently at the park?
I think you can buy Genie+ ahead of time (at least it was offered when I looked at tickets this weekend) but you can't start using it until you've scanned into the park.
From the Disneyland website: Disney Genie+ service is available for purchase as an add-on with new ticket or vacation packages before your visit, or as a single-day purchase for existing ticket holders through the Disneyland app on the day of your park visit after park entry.