An unlawful assembly is three or more person 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, knowing or having reasonable grounds to believe that it is such, commits a violation of this code. (See Title B, Chapter 2, § 1.28 or Iowa Code § 723.2.)