I thought I found the official name for band members who don't really contribute musically and are there just for the show - see for example the "invisible man" who's mostly responsible for drinks/drinking in this video:

https://www.youtube.com/watch?v=QwkP5k3nmaE

So I saw an interview where I thought the phrase "dead microphone" was used to describe this role.

Of course when I google'd the name, I mostly got #Linux bug reports...