Ok a few things to try,
Try it out with one player/friend
Start Google+ Hangout from inside Roll20
click *Virtual Tabletop* ->find your campaign - Click on -> *view details* link then click on the* Launch in Google+ Hangout*
check your players/Friends -> all have Google + accounts
check that they are logged into thier Google+ accounts
that they accept the invite from there.
Check they clicked on the ROll20 APP tag at the bottom of hangouts to activate it ( think you only need to do this the first time, than i think it becomes one of thier google hangout APPS at the top)
Good luck
