A sideman is a professional musician who is hired to perform or record with a group of which he or she is not a regular member. They often tour with solo acts as well as bands and jazz ensembles. Sidemen are generally required to be adaptable to many different styles of music, and so able to fit smoothly into the group in which they are currently playing. Often aspiring musicians start out as sidemen, and then move on to develop their own sound, a name, and fans of their own, or go on to form their own groups. Wikipedia.