That's true. It is technically sold out, however, if you really want to attend and it's a small enough group, you may be able to get a Rez the day of the party.
First, stop in at DHS Guest Relations and check. You may or may not strike out there.
If you do, go to the GMR party venue about three hours before the JBJB show while party set-up is going on and find one of the five red vested JBJB party CMs. It has to be one of those CMs because they are specifically told if walk-ups are available for that night. If they are being taken, a leader will book it right there. What are your chances? Slim, but possible.
If you're told that walk-ups are not available, but you really, really want to attend, don't give up. Politely, and while flashing your best puppy dog eyes, schmooze the CM. If you have little ones, have them use their sad eyes too. CMs are suckers for that kind of thing. Well, at least one I know is. They can and do overbook it.