WhatsApp groups may not be as secure, as one would think. Some users have reported that Google indexes group invites, if they were created through the “Invite to Group via Link” feature. So groups are actually very accessible to any person who knows how to use Google.
According to WhatsApp, invites can be found via search engines, online if they were posted on the open Web. Also, WhatsApp states that invites can be disabled at any given moment by admins. But it’s not up to Facebook, what Google and other engines index and what not.