Iowa Code § 723.2: Unlawful assembly.

An unlawful assembly is three or more persons assembled together, with them or any of them acting in a violent manner, and with intent that they or any of them will commit a public offense. A person who willingly joins in or remains a part of an unlawful assembly, or who joined a lawful assembly but willingly remains after the assembly becomes unlawful, knowing or having reasonable grounds to believe that it is such, commits an aggravated misdemeanor.