Getting more testers mid-test

If you want more than your original plan's testers, use "Get more testers" in your account menu. It moves your existing test to a bigger tier, charges only the difference, and your existing testers' day count keeps running uninterrupted. The new testers join the test already in progress and start their own opt-in clock from the day they're added — Google counts each tester's continuous days individually, not from your test's original start date, so they need their own full run before they count toward the requirement.