"If you don’t have a Google+ account you can start or join a limited hangout, which only includes the option to hang out with 1 other person. If you’d like access to all Hangouts’ features including the ability to hang out with up to 9 people, share your screen, and use cool Hangouts’ apps, you can upgrade to a free Google+ account at plus.google.com"
from https://support.google.com/chat/answer/161986?hl=en