Go back to the article and search for "If you're interested in trying it out", there's a link that allows you to signup for the waiting list.
We've been ramping up our invites from the waitlist — our Slack community has over 18,000 members — but we still are only a small fraction of way through. We've been really overwhelmed with the demand and have been scaling our team and processes to be able to meet it.
We can also often accelerate invites for people who do have a specific application they'd like to build. Please feel free to email me (gdb@openai.com) and I may be able to help. (As a caveat, I get about a hundred emails a week, so I can't reply to all of them — but know that I will do my best.)