The "Become a Tester" Button: When they click that link, they must click a blue button that says "Become a Tester"(or "Join Program"). If they don't click that, they won't be counted toward the 12-person requirement, even if they have the app installed.
Existing Internal Users: If they already have the Internal Testing version installed, clicking this link "upgrades" their status to a Closed Tester. They might not even need to download a new file; the Play Store will simply acknowledge their new status.