Here is a summary of the process:
1. In your web browser address (URL) box, type g.co/hangouts.
- email invite
- call a phone (use keypad icon)
- share hangout link (e.g. via chat session)
3. Enter a phone number (or a contact name)
At any time after starting the Hangout using g.co/hangouts, computer or phone users can be added by clicking the "people+" icon to get the window shown in step 2 above.