There is agreement on many aspects of joint authorship, but not all. It is generally agreed that in the United States, joint authors are tenants in common, meaning all own an undivided proportional share in the entirety: 2 authors 50%, 3 authors 33 1/3% etc. Joint authors are not liable for infringement for using the work themselves, and are subject only to a (state law) duty to account for any profits earned. Joint authors can engage in non-exclusive licensing without the others' permission and the licensee is immune from suit by the other joint authors. A joint author may sell its proportional interest without the permission of the others, but cannot engage, solo, in exclusive licensing (which is the same thing as an assignment of the right licensed).
It is also settled, contrary to Nimmer, that to be a joint author one must contribute a modium of expression, and not just ideas, and that if one does so, the proportional share is calculated per capita and not on the extent of the contribution made, meaning that if there are two joint authors and one contributed 10% of the expression that joint author nevertheless owns a 50% interest. It is also agreed that joint authors can vary their proportional interest, but that such an agreement is a transfer of copyright, requiring compliance with Section 204.
One area of disagreement is Goldstein's view that to be a joint author one must contribute expression which can stand on its own as a copyrightable work. That view is based on a misreading of the Second Circuit's Childress v. Taylor opinion, and also reads out of the statute inseparable joint works, leaving only interdependent ones. One unsettled and probably insoluble problem is a joint work created by a private individual (or company) and an employee of the United States acting within the scope of his or her employment. Section 105 excludes from protection works of the United States government, but what happens when only one author is a government employee? This problem also arises with bootlegs, for example if Trent Lott were to sing in a barbershop quartet at the Capitol with private individuals.