WhatsApp right now is not an alternative for services like MailChimp or Newsletters.
WhatsApp allows to send messages to 256 users simultaneously using “Broadcast lists“. In a newsletter is very common to have thousands of users. This is a very narrow limit from WhatsApp, but as you know WhatsApp is all about real conversations, no Spam. If you have less than 256 users, you can use WhatsApp as Newsletters and send your WordPress news to your users.
To send a message to this list, they user needs to have you in his/her contact list. If the user do not have you in her / his contact list, your message will not hit their WhatsApp inbox. This is a reason to insist to your contacts to add your number to their contact list.
Once you have your Broadcast List ready, you can create your message and send it.
Please, avoid Spam always. Send meaningful messages to your lists and create value to your customers.