To access the new feature, update your WhatsApp, then go to Settings > Account > Privacy > Groups. WhatsApp gives you three options of who can add you to groups: Everyone, My Contacts or My Contacts Except.
If you set it to everyone, you will be added by anyone with your phone number. If you set it to ‘my contacts’, only your contacts will be able to add your number. If you set it to the third option, you can shut out specific contacts from adding you to groups. If you enable this feature, group admins who can’t add you to a group will have the option of inviting you privately.
Initially, it was said that these invitations last for 72 hours. I believe the third option will become very popular as people will try to make their WhatsApp accounts more private than ever before. This will be a godsend to those people that have suffered in the hands of being added into groups that they do not want.