He wants to join an organization, that's it. If he commits a crime (such as killing someone), try him, convict him, and put him in prison.
People can join the Communist Party USA, and it's not a crime. If they commit arson, that's a crime. But, joining the CPUSA isn't a crime.
Democrats and Republicans spray paint slogans on buildings or steal yard signs. But, defacing a building or theft are crimes, not joining the Democratic or Republican parties.
Two decades ago, I worked with a gay rights organization. One night, someone slipped under our door a pamphlet titled "The Bible justifies the killing of homosexuals". If we had found the literature dropper, should the police have arrested him or her on the grounds s/he advocated violence? No!