It is unlawful for three or more persons to assemble 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. No person shall willingly join in or remain part of an unlawful assembly, knowing or having reasonable grounds to believe it is such.
(Iowa Code § 723.2) (1999 Code, § 40.06) Penalty, see § 10.99