setting up a game with Google Hangouts

I'd like to play a game with Google Hangouts and use the "Looking For Group" tool in the Roll20 website. I'm wondering how it is commonly done. I believe if someone joins my game via the Roll20 website, they still have to somehow join the Hangout via Google.  Are your Hangouts "public" or do you invite people? If you invite people, how do you invite people who find your game via the Roll20 web site? 

You would need to at a minimum have an email address for everyone in your campaign. You could do this via the campaign forum, ask everyone to either drop a link to their G+ profile or an email address. Then when you start the hangout use that information to invite those specific people to the hangout.