It is possible for users to join Hangouts video call's without Google account of their own. This can be done by so called Guest mode. Guests can join a Hangouts video call with a link that has been shared to them:

This requires that external users are allowed to join the Hangout video call. If needed, this setting can be changed from "Invite People" or by clicking the name of the video call from upper-right corner:

Guest opens the link in his/her browser (Google Chrome preferred), types his/her name and requests permission to join the video call:

After this the creator of the video call receives a pop-up window to either accept or decline the request. When accepted the guest automatically joins the video call.