Late to the party. W.r.t. Artem's quote -
"Hangouts is terrible about notifying about new chats from people not on the list"
For first few months, I didn't even know that existed. One fine day when I had too much free time in my hand, I was going through each of the settings in hangout when I stumbled upon "Hangout invites" where I had ~50 pings from unknown people. my reaction -> :-0