I had to do this recently as my sister, and then a few weeks later, my brother decided to come. This was well after I had booked fastpasses 60 days out for the original party of five. The person needs to have valid park admission in order to make fastpasses. We had my sister and brother buy their tickets online through Disney because we got the ticket number right away. Go to
MDE and add them. You will need to link their ticket # to the account.
Once the ticket is linked, you can add fastpasses to existing fastpasses. Their may not be availability at the same times as the existing ones. For example, I couldn't get my brother on Toy Story at all. A couple of our fastpasses had overlapping times so we could go on all together. I also could not get my sister a fastpass for Toy Story with us, but their was a later time available so she went alone.
The process for adding a fastpass to an existing fastpass is easier than I thought, follow the prompts. If MDE can't copy them exactly, they will give you one to three options of choices, choose one (it won't change them all-just the person you are trying to get fastpasses for) and you can attempt to adjust the times or attractions later.
We were very lucky that 7DMT opened up many choices for our dates in November. Previously, I couldn't get my brother or sister on that ride at all. Times opened up and we all got to ride together...twice! So keep checking.
ADR's-not sure. We didn't have any except for a FP at BOG which automatically added them because they were added to the room reservation which the BOG fastpasses are based on.