With the Broadcast List feature, you can send a message to several of your friends at once.
Broadcast Lists are saved lists of message recipients that you can repeatedly send broadcast messages to without having to select them each time.
To create a Broadcast List:
This will create a new Broadcast List. When you send a message to the Broadcast List, it will send to all recipients in the list who have your number saved in their phones' address book. The recipients will receive the message as a normal message. When they reply, it will appear as a normal message in your Chats screen; their reply will not be sent to other recipients in the Broadcast List.
WhatsApp Support Team