Depending upon what type of ensemble it is, oftentimes a player doesn't really have to audition. The leader has heard of him, and just asks him to join. Surprisingly many rock bands have auditions, especially to replace a player who has left.
And of course all major symphony orchestras, and I presume concert bands, audition for vacancies. There are a lot more openings for string players, whereas Tuba and timpani don't come up that often because there's only one per orchestra.
In my own case I got the gig for the first name band I was in without realizing it was an audition. I thought I was simply getting hired to play extra on some recordings as a one time session musician...