Using the Broadcast List feature
A Broadcast List allows you create, save and message a list of contacts instantaneously. Recipients receive your broadcast message like a regular message - directly in the individual chat. Recipients also do not know who else received the message and do not know that it was sent as part of a Broadcast List. When they respond, they only respond to you individually. In this sense, it works like the bcc (blind carbon copy) function in an email.
In order to be able to send a message to specific contacts in your Broadcast List, these contacts must have your phone number saved in their address books. This allows our users to control from whom they wish to receive broadcast messages. We work hard to curb spam in WhatsApp so that everyone has a positive messaging experience.
WhatsApp Support Team