Developing musicainship and specific performance skills along with arranging, and composition.  Soloing, imporvising, and learning alternitive styles of music are parts of the modern jazz ensemble.  A player music contribute more than just good part playing - the jazz musician has to bring musical ideas to the work environment.  Adding an alternative ending or introduction to a song, creating or extending solo sections within an extsting arangement, reharmonizing, changing an arrangements genre; these are all parts of being in a small group/jazz ensemble.