A more thorough explanation:

Joint participation 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. It is also known as joint activity.

For example, if a private citizen works with a police officer to conduct a search, and the search violates someone's civil rights, both the private citizen and the police officer can be held accountable for joint participation in the violation.

Joint participation is determined by the symbiotic-relationship test or nexus test, which looks at the relationship between the private person and the government entity or state official.