Make sure that they don't have already installed Whatsapp. If they don't
having Whatsapp already then you need to download it from iTunes search
like Whatsapp for free because iTunes Offers sometime free apps.
Now Install Whatsapp in iOS. After the installation, Activate your mobile number and verify it.
After the verification, now use your Activated number on your own device
and Check your Service time it is extended to Lifetime.