If I start a new meeting I can change the video background and select from a whole bunch of provided backgrounds. Also, there is the option to add new backgrounds.
But if I join a meeting via the "Click here to join the meeting" link from an invite, the only option available is to blur the background, there are no other background choices, and no option to add a background.
Other things that I noticed:
the first background options panel (start a new meeting) has a dark background, and the second background options panel (join a meeting) has a light background
when starting a new meeting, the blur background option shows the silhouette of a person against a background split between two shades of grey. for joining a meeting, there is no silhouette of a person, and a single shade of grey.