With the Broadcast Message feature of Whatsapp, you can send a message to several of your friends at once.

To broadcast a message on Android you must have a Broadcast List or create a new one as follows:

Open WhatsApp.
Go to the Chats screen > Menu Button.
Tap New Broadcast Lists.
Tap + or type in contact names to choose recipients from your contact list.
Tap Done.
Tap Create.

This will create a new list of broadcast recipients. When you send a message to the Broadcast List, it will send to all recipients in the list. 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 send to other recipients in the Broadcast List.

Note: Only contacts who have added you to their phone contacts list will receive your Broadcast Message.

http://www.mobilitaria.com/2014/10/send-a-broadcast-message-on-whatsapp-for-android-z10-iphone.html