It's the white phone on a green speech bubble icon.

If this is your first time opening WhatsApp on your phone, you'll need to set up WhatsApp first.
 It's in the bottom right corner of the screen.

If WhatsApp opens to a conversation, tap the back button in the top left corner of the screen first.
 You'll find this option at the bottom of the screen., It's in the middle of the pop-up window here., You may have to scroll down to find it.

Any names that appear here are non-WhatsApp users from your iPhone's Contacts app.
You can use the search bar at the top of the screen to search for a specific contact.
 It's in the bottom right corner of the screen. Doing so will open a New Message window with a link to WhatsApp.

If you tapped more than one name, you'll see Send Invites instead.
 It's the green or blue icon in the right side of the text window near the bottom of the screen. Doing so will send an invite to your selected person (or people); if they download and join WhatsApp, you'll be able to contact them through the app.
Open the WhatsApp app.
Tap Settings.
Scroll down and tap Tell a Friend.
Tap Message.
Tap your friend's name.
Tap Send 1 Invite.
Tap the send arrow.
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