Greetings, Mary!
Thanks so much for stopping by the Disney Parks Moms Panel with your question. I'd be happy to assist you with planning your upcoming vacation. It sounds like you are going to make some magical memories with so many friends and loved ones at the Happiest Place on Earth!
While every ticketed member of your party is more than welcome to create an account for the
Disneyland app, it isn't completely necessary for them to do so even if you will all have the amazing
Disney MaxPass feature on your tickets. As you said you can easily link tickets to an app account. For example if you create a Disneyland account on the app, you can scan everyone's ticket to your account to link them. Once they are linked to your account with Disney MaxPass already on their tickets, you can go ahead and start planning your first Disney FASTPASS selection with Disney MaxPass right after you enter the park. Just select the members of your party who will be a part of your first FASTPASS selection. If members of your group want to split up and make Disney MaxPass selections on their own, it will benefit them to create their own app account (and link tickets to those accounts) so they can make their own FASTPASS selections with their party.
You will also want a Disneyland account on the Disneyland app if you choose to mobile order food and beverages during your trip. This is my very favorite feature on the app as it has saved my family SO much time waiting in food lines over our vacations. I love that I can place my order while I'm in line for an attraction and let the Quick Service restaurant know "I'm here" when I exit the attraction and I'm ready to eat. It definitely lets us fit more wonderful experiences in our day.
I hope this helps, and please return if you have any other questions. Have a Zip-a-Dee-Doo-Dah day!
-Jennifer