He is now to be among you at the calling of your hearts. ...
The union of your spirits here has caused Him to remain,
for whenever two or more of you are gathered in His name,
there is love.
These words by Paul Stookey from "The Wedding Song (There Is Love)" can provide some guidance, with a bit of modification, when more than one person assists in the development of music. In a case published earlier this week by the Seventh Circuit, a person who assisted a principal composer's development of a song, entitled "Wonders of Indiana," was awarded co-authorship (and co-ownership) rights. As such, it can be said that whenever two or more are gathered to develop a song, there is -- usually -- co-authorship (and co-ownership) rather than love. This is the case even if a "contributor" provides unequal or marginal contribution to the development.
A co-authorship exists when the parties intend to create a joint work (this element can be satisfied if the parties intend that the resulting work incorporate the jointly developed elements), and if the marginal contributor's contribution added more than mere general ideas but includes concrete, creative expression.
A primary author should be careful when seeking substantive developmental assistance from another person, for the resulting work may be jointly owned. To avoid this result, the primary author should require that the marginal contributor sign an agreement disclaiming authorship and ownership interest, and acknowledge that the primary author is the sole author and owner.