USJ Timed Entry for Nintendo World Question

So for Universal Studios Japan, I know to get into Nintendo World you either need to get there very early and walk-in or get an "Area Timed Entry Ticket"; worst case scenario being trying your luck with the standby lottery. Me and my family are planning to go in January and definitely want to check it out. In my research, I read that in the case of Timed Entry, all Studio Passes must be on the app before creating a group and selecting a time slot to enter the land.

Now the issue I'm concerned about is that my brother and I plan on going to the park earlier than our parents and brother because they usually aren't keen on going to a theme park super early. Knowing this, our plan is to get to the park before them and get a Timed Entry slot for all of us later when we are all together.

My question is, in order to get a Timed Entry slot, do all the tickets need to be scanned into the park before a time can be selected, or can a couple members arrive earlier and secure a later time slot for the whole group? Just want to know so that in the case the whole group has to be in the park, I can tell my family to plan on getting up early.

(P.S. just an aside, I noticed that Harry Potter also has a timed entry, but does anyone know if that even get busy enough for them to use it?)