Haven't been following the recent conversation with regard to Grand opening date and all that, but when a theme park announces an "Opening date" for a land, attraction, or park, that's the day it's open to all guests. Passholder previews, press previews, cast/team member previews, employee previews, etc are all done prior to the "Grand Opening." That's the way it's done. Often times the media gala will be held the day before the official opening.
So when Universal says the grand opening date is June 18th, that means that anyone who's in the park is
allowed to enter Wizarding World and ride any attractions that are functioning. Whether or not you'll be able to get in due to capacity problems is another story, but they won't tell you, on June 18th, "we're sorry, this grand opening was for hotel guests only" or something to that effect. Otherwise they'd advertise the opening as an exclusive opening for specific types of people only, not as simply a general grand opening.
Unless they have unexpected delays with attractions, June 18th is the "open to all of the public" date, capacity not withstanding.
To clarify - I personally believe that park capacity, in addition to Wizarding World capacity, are going to be maxed out on June 18th. You will have a tough time even moving around at IOA. But if you get there early enough (how early I don't know), you will absolutely not be turned away because you don't have a special lanyard. Publically announced "grand opening dates" are just that. The day it's open to the public. Having FJ, the centerpiece and only true attraction in the Wizarding World, closed on "opening day" would be a colossal nightmare for Universal. They would not, under any circumstances, announce June 18th as the official opening date of WW if FJ was going to be off limits to people without special access. But again - good luck riding it on that day in light of the masses of people that will be there.
--- Update ---
Scroll back, I just posted pictures. This was the day before EE's QUOTE "Grand Opening". Major theme park Grand Openings are NOT, and NEVER HAVE BEEN open to the public in recent memory. I promise you that this is truly the case with this mega attraction. I am not trying to be a jerk... but my pictures above speak for themselves. You couldn't get NEAR EE.
Grand Openings are always open to the public. Media days, preview days, etc are (almost) always closed to the public. June 18th is the public opening of WW. If the FJ attraction is fully functioning, and you get there early enough, you will not be turned away for not having the proper authorization to ride. You can, and likely will, be turned away for getting there too late or for capacity reasons. Sorry, Teebin, I like and respect your posts, but you're wrong on the terminology on this one.