A Joint Work is legally defined as “a work performed by two or more authors with the intention that their contributions be merged into inseparable or interdependent parts of a unitary whole.” 17 U.S.C. §101
A joint work has more than one “author”. That means that the rights to the intellectual property are shared jointly.
When two or more people come together to create a single work that is the result of there collective contributions, the work is considered joint. That means, all of the contributors have rights to the property.
That means there are two authors of a work. Each author contributed work (e.g. an idea or coding skills) to create one work. A work that would not exist if both parts were not involved. In many business endeavors, creation of intellectual property is the creation of a joint work. That means that, unless the ownership of the work is in writing, all of the co-founders own rights in the intellectual property.