I just wish that I knew how to get in on these NAGBs.
It seems pretty straight-forward to me.
1) Be on good terms with the organizer. For Royal Princess Casino and Sunset Beach, that was
@Lemonzest. For Big Top Poker, that was (I think)
@Chicken Rob . For Empress Star, Aurora Star, and Pacific Star, I don't think the organizer has been publicly named; I don't know who it is, but it's not hard to find out who does know. As for who's going to be the next one to organize something like this... ¯\_(ツ)_/¯ You could either take your best guess based on seeing a) who's done it before or b) who seems like the kind of person who might try it and succeed, OR maybe just try to be on good terms with everyone.
2) Be known for making multiple large transactions and having them all go smoothly; mostly purchases, but probably a few sales would help as well. You probably won't be invited to participate if the organizer isn't confident that you'll pay promptly when asked to.
3) Be known for being willing and able to pay a high price for highly-demanded chips. It's possible the next custom buy will be offered to participants at close to cost (in which case you'd be getting an enormous financial windfall) but it's more likely they'll be offered to participants at close to market. If so, you probably won't be invited to participate if the organizer isn't confident you can and will pay the market price.
4) Be known for being the kind of person who can keep their mouth shut. These kinds of purchases can probably be wrecked if they're widely known about before they're complete. You won't get an invite if the organizer thinks you might talk about it.
and then finally:
5) Ask! Not here. Not in public. Not in general. PM specific people and make a specific request. Identify people you think might organize another purchase and ask them to include you if they do.
More generally, though - do a lot of business with a lot of people. Buy sets; sell sets; make inquiries and offers through PMs. Ask around looking for chips or for buyers; even if the people you talk to aren't interested, maybe they'll connect you to someone who is, and at the very least they'll get the idea that you are an active trader. If you're nice to the people you talk to and don't annoy them, they'll probably think well of you, and may keep you in mind when the next custom purchase starts getting fired up and the organizers start asking "Okay, who can we invite to participate now?"
... or at least, that's what I surmise, based on what we've seen in public so far.