Can I buy Genie+ or ILL for only a few members of my party or do I need to add them to all attached tickets? Will my tickets appear on my app on the appropriate day? What else can I use as a ticket if I don't have my phone upon entry? Room key?”

Hiya Emmanuel! Welcome to planDisney! 

Oh boy! I'm excited to hEAR you will be traveling to the Walt Disney World Resort to start off the new year! You'll be arriving in plenty of time to still take part in the 50th Anniversary Celebration, which is slated to last until the end of March. It seems like we have a lot to discuss with your questions, so let's dive right into it!

1. I have great news for you! When purchasing both Disney Genie+ and Individual Lightning Lanes, you can pick and choose which Guests will be using them on a specific day. If there is a park day where only a couple of you need to use the service, the rest of the party can opt not to do so. This helps save and provides flexibility for each Guest. On a recent trip, my wife and I decided to purchase Disney Genie+ for just us, since my daughters were not going to ride many of the thrill attractions that we were planning for the day. We were able to ride the attractions that they wanted to by waiting in the standby queue. We then made Genie+ reservations for us and used the Rider Switch Service as well. 
2. Your park tickets should already be shown in your My Disney Experience App if you have purchased and linked them. To check, log on to the app and select "Tickets and Passes". They will be listed there if they were linked. Your park passes to the park you plan to visit each day will be shown by selecting "Future Plans". You will have needed to make theme park reservations to the park you plan to visit each day. If you haven't done so already, select the "+" at the bottom of the app followed by "Make or Modify Park Reservations". You'll be able to make them here for the dates that your tickets are valid.
3. You'll have a few options when it comes to entry into the parks each day. You can purchase a MagicBand or MagicBand+ upon arrival at the Walt Disney World Resort. MagicBands have plenty of fun uses, including using them upon entry into the parks. You can also use them to pay for food and merchandise around the Resort and as a resort room key if you are staying in a Disney Resorts Hotel. Lastly, you can stop by the ticket booth located outside of each of the four theme parks on your first park day to pick up a ticket card. You can use it for entry into the parks each day. You'll be able to use the same card throughout your stay.

Emmanuel, I hope this information helps you plan for your upcoming visit! I'm sure you are going to have a magical time!

See ya real soon!
Greg