A more thorough explanation:

Joint activity refers to a situation where a private person works together with a government entity or state official to perform public functions. This means that the private person is subject to claims under civil-rights laws.

For example, if a private citizen works with a police officer to arrest someone, and that person's civil rights are violated during the arrest, both the private citizen and the police officer can be held responsible for the violation.

Joint activity is also known as joint participation and is determined by the symbiotic-relationship test or nexus test.

Other legal terms related to joint activity include joint tenancy, joint tortfeasors, and joint trustee.