Google+ Hangouts in Google Calendar
Creating events with Google+ Hangouts
You’ll see the link to Add a Google+ hangout on the event page for any individual events you create, and clicking on it will add a hangout to the event. Guests with edit access to the event (either through ‘Guests can modify’ being enabled or shared ownership of the calendar) can add a hangout to the event; guests without edit access will not be able to add a hangout to the event.
You can also choose to have hangouts automatically added to events that you create. To do so, go to Settings (gear icon Settings) and select Yes next to Automatically add Google+ hangouts to events I create. This option is disbaled by default. When you enable it, every event you create using Google Calendar on desktop will have a link to a Google+ hangout.
If you don’t want a hangout for a particular event, click Remove next to the hangout link.
Guests of an event with a Google+ hangout should click Join Google+ hangout in the event bubble or event page to enter into the video call with other guests.
You must be a Google+ user to join a hangout. If a guest to an event with a hangout is not currently a Google+ user, they'll see a suggestion to sign up for Google+.
Additionally, some domains may not allow Google+ hangouts, in which case you’ll see "Google+ Hangouts aren't enabled for your organization" displayed on the event page in the "Video call" field.