background props
also BGP; placing oneself in the background of another person's picture (could be stranger or a friend), usually striking some strange pose or making some other gesture, unbeknownst to the subjects or the photographer
Who is that guy getting his BGPs in all of my pictures?
background prop
A person who is invited to a party or other social activity simply as filler, and isn't really liked much by anyone there.
After Danny got into an argument with a friend of the host, she ended it by telling him that nobody wanted him there. He looked around and found no support, and so he left. He knew then that he was invited simply to be a background prop.