If you head over to the People & options setting of a group chat on Hangouts, you'll see — if the feature is live for you already — a new toggle to enable Joining by link followed by an option to share the link to other apps. It may not seem like such a big deal, but this should allow users to invite others to conversations without having them in their contact list on Hangouts first. You have to have a point of contact with them though, because you still need to share that link somehow. WhatsApp was reportedly going to let you print group links as QR codes, which might make even more sense if you want your group to be more accessible to users you can't yet contact through your phone.
You can download Hangouts 11 from APK Mirror if the update isn't live for you yet on the Play Store, and then check to see if invite links are showing for you. They might take a few days to roll out to everyone.