A joint work is a work prepared by two or more people with the intention that their individual contributions be merged into a single whole body of work.
The creators of joint works are equal co-owners of the copyright unless they have agreed to the contrary. Whoever contributed to the work has an equal claim to the entire copyright as a joint author. The copyright is not divided up according to who did what.
Each joint author owns the copyright and each author has rights to use or license the work as long as they divide any revenue earned. One joint author cannot prevent another from using the copyrighted work.