I booked our Sept trip on 07/19 and held off calling until today. I did attempt to find the reservation on the Marriott website (under My Trips) but it never appeared.
We called this morning to add my daughter and didn't have any issues. They found the reservation (and verified all the details) and added her.
It was so shockingly easy/quick that when I got off the phone I wondered if I asked all the right questions.
I didn't ask for the code to add it to
MDE yet... I figured that I'd give it another day or so, call again, verify everything one more time, and then ask for it.
Hoping you have the same experience; good luck.