Despite the cliché of the genial composer slaving away alone in a lonely garret, many composers put their music together in more collaborative environments. Sometimes this can be a fairly egalitarian arrangement, as in the Cage/Harrison Double Music (see this item on co-composing); in other cases, a single composer is identified with the work to which other musicians contributed "work for hire." The advantages of working with such assistance can be great. Jean-Baptiste Lully was said to have composed the melody and bass line and then handed the materials on to a team of assistants who — in the manner of an assembly line — efficiently filled in the interior parts in the five-voiced string texture according to the master's specifications. Giacinto Scelsi recorded improvisations at the piano or an electronic ondioline and then hired musicians to "transcribe" the recordings into scores, work that was often highly imaginative in character.
(In many traditional music cultures, attribution to a single composer is generally unknown. In Central Java, for example, it was conventional to identify a composition with the ruler of the court with which the music was associated. I've often wondered if we are a not a bit hasty in automatically ascribing musical works in the early western canon to individuals rather than to those communities within which the individuals worked.)
Sometimes collaborative work can create controversies with regard to attribution. In Scelsi's case, one composer who had worked extensively for Scelsi as a transcriber came forward after Scelsi's death with the claim that he had been the composer of works published under Scelsi's name. However, the contractual relationship was clear — it was work for hire — and the work produced in that relationship could, in no way, confused with work the hired transcriber chose to present as his own; moreover the works in question were stylistically consistent with the work Scelsi produced with other assistants. The contribution of these assistants was not insignificant — indeed, in several cases, it approaches the virtuosic — but the constant element is clearly the conception and style of Scelsi himself.
A recent weekend read was Mike Barnes' s Captain Beefheart: The Biography, which interested me because of a personal geographical connection rather than a musical one. It turns out that the book really ought to have carried the title The Magic Band: The Biography This is because the narrative is carried more by the description of the collaboration among various configurations of musicians and producers through which the music they made was produced than by the biography of the Captain, Don Van Vliet, himself. (Indeed, much of Van Vliet's biography — and certainly almost anything about his character — is obscure or speculative and not least because he's no longer talking, and when he was talking, he was notoriously vague and unreliable.)
My knowledge of the workings of popular music is limited, but I suspect that collaborative composition of the sort practiced in the Magic Band is a standard practice: someone brings in some text or some riffs or licks or a fragment of a tune or a sequence of chords and songs get assembled, bricolage-ishly one might say, with each of the individual musicians contributing their own takes and turns on the material. In this case, the raw material was often taken from recordings of Van Vliet pounding out some patterns or fragments on the piano (or, sometimes, singing or even whistling them) and then, working with each of the musicians, gradually working this into functioning parts of a larger ensemble continuity. In practice, one of the musicians usually functioned as a musical director for the band, transcribing recordings and coaching the other players, to which instruction Van Vliet would provide additional suggestions, singing, whistling, or offering metaphorical expressive notes.
However collaborative the enterprise was, and how limited Van Vliet's contribution was to many details, almost all aspects of idiomatic (and, when required, extra-idiomatic) instrumental technique, and much of the basic continuity, it is also perfectly clear that Van Vliet was the composer here: it was his aesthetic at work, he defined the style, he decided what went into a piece or was left out. Moreover, he was the constant factor in a band that otherwise had several complete turn-overs in membership and the music made by band members aside from their work with Van Vliet was (an is) distinct. Yes, he had plenty of help — and superb helpers, at that — but that doesn't diminish the compositional accomplishment.