Whatsapp has changed or updated its group settings to include "everyone" by default. This will allow people you don't know to add you to a group without you knowing. Before the change, only telephone numbers saved to your contact list would have been allowed to add you to a group. And, if someone whose telephone number is not in your contact list attempts to add you to a group, he/she will be prompted by the app to send you a private invite through individual chat. This would give you the choice of joining the new group or ignore the invite which would expire in 3 days.
If you are concerned about your privacy and do not want anyone to add you to a WhatsApp group without your consent, you may change your Whatsapp Group Settings from "Everyone" to "My Contacts".
Changing Your Default Whatsapp Group Settings