TestFlight public link doesn't accept new beta testers

Well, after writing this, I decided to check the build state and I noticed that the build was missing a step:

After notifying the testers, the link now accepts new testers. The link shows: "To join the Mealcard beta, tap the link on your iPhone or iPad after you install TestFlight."

If, like I found, you're finding clicking "Notify Testers" still doesn't work as outlined in Ricardo's post, give it 10 minutes to propagate across Apple's networks.