• If a group admin who isn't in your phone's address book tries to add you to a group, they'll get a pop-up saying they can't and be requested to touch Invite to Group or hit Continue, then send to send a private group invite over an individual conversation.
• A WhatsApp user who isn't in your phone's contacts won't be able to invite you to a group. With the exception of: Except for particular individuals you specifically want to exclude, this option allows everyone in your phone's contacts to add you to a group.

Read More: Why can't I add a person on Whatsapp's contact list to a group?