They won't be open to the public... but it is basically a dress rehearsal of being open & operational.
In some cases, like say a restaurant, they will do a mock opening with the staff... half of the staff will do their designated job, while the other half pretends to be the guests/customer. Then they switch. I know at the Burbank Steak 'n Shake did this last month, but on two days (Friday/Saturday) before they were actually open, they allowed real customers to come in & eat for free to give the staff some legit experience dealing with guests.
I don't know if that is how they will do it with Potter, but I figured it would help give you an idea. As for any park guests getting to enter Hogsmeade during the mock opening, I would say it is highly unlikely. If anything, they'll probably let other Non-Potter USH TMs into the new land during the 8th-11th to help serve as a TM preview.