When I read this I realized this is a good question as I haven't done this yet in the new app.
I'm going to need this when my reservations get wiped out for the new dining.
I found this in the New App thread....I can't try it as I don't want to mess up my reservations, but this was someone's suggestion:
You click book dining…pull up the MDR you want, look at the date, see top right corner. Person outline with a +. Click on that and add the other person’s booking number. Select the person/persons you want to add to your reservation.
Good luck!