I just found this warning and checked my settings, and they had changed to "Everyone", so I had to alter them. Perhaps you might check your settings too? — Maria
If you use WhatsApp, then a recent update has been made you may not be aware of. 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 can include scam messages, loan sharks, etc. You can change its default settings as follows, within WhatsApp —
Go into Settings
Go to Account
Go to Privacy
Go to Groups
Change from "Everyone" to "My Contacts"