Kith
Get Kith on your phone.
Kith is where you already are — on your phone, and on WhatsApp. Pick your device below; setup takes about a minute.
On iPhone: the link opens Apple's TestFlight app — install that first if you don't have it, then come back. If the button does nothing, open this page in Safari rather than inside your messaging app.
On Android: the link asks you to accept the test, then hands you to Google Play to install. Accept with the Google account your phone's Play Store uses.
Kith is invitation-only. Sign in with the email address you were invited with — another Google account won't be recognised.