I just did this recently, but as we have been moving I cannot guarantee that my memory on the process is entirely correct. 🙃
I think I just went to the dining reservations (even though I am on the Sun in October and can't yet make reservations) and picked a random restaurant. Then I hit the button with the pencil and either used the search function or the little person with a plus sign up in the top right corner and added our friend. It does look like when the time comes to make the dining reservations, I will have to hit the button with the pencil and check her name if she will be joining us for that particular meal.
Hopefully, I more or less remembered the correct procedure.