You will be asked if you want to accept or decline a video if one or more of the people in the group call are not on your contact list. Clicking Accept allows you to start viewing their video.

You also have the option of adding them to your contact list at the click of a button. If you accept and add them, you will be able to view their video automatically in all future group video calls, as well as gaining all the other benefits that come with adding a person to your contact list.