There is NO way that the reservation system will be set up so that you can't make a reservation until you enter the park. NO WAY. The whole purpose of this system at
Disneyland is to control crowds onsite.
I suspect they will simply have a sign up online where you sign into your Disney account (I think having a Disney account will be required and that is how they will track it) and then reserve a time slot, naming the number of guests who will be with you. You will get a print out or it will show on your app, and you will show that to gain entry to the land on your designated day. If people make a reservation without a ticket, they will likely be told that a ticket will be required for regular park admission on the day of their reservation. I suspect the printout will say "Not a theme park admission ticket" in huge, bold lettering on the top. If those people don't show up at their designated day/time, too bad. I think they will build in a certain percentage of no shows into the system, likely overbooking it somewhat.
What I think people WILL be able to do is, book a time slot once PER REGISTERED GUEST. Like, in my household, I will be able to book a time slot for "4 guests" under my account, and my husband will also be able to do so, on a different day. They cannot require ALL guests coming into the land to make their own reservation (like, kids can't create a Disney account) and they have to account for large or extended families where not all the adults will have a Disney account (grandparents, etc).
Once you make the reservation online, it will likely just be "Registered member plus X number of guests." I think they will make it non transferable, or people will sell them online. So, each reservation will likely be linked to one Disney account.