WhatsApp has changed its group settings to include “everyone” by default so people you don’t know can add you to a group without your knowing. These people may include scam messages, loan Sharks, etc.
You can change its default settings as follows:
1. Go to WhatsApp:
2. Go into Settings
3. Go to Account
4. Go to Privacy
5. Go to Groups
6. Change from (Everyone) to (My Contacts)
https://faq.whatsapp.com/general/security-and-privacy/how-to-change-group-privacy-settings
https://www.internetmatters.org/parental-controls/social-media/whatsapp/