According to the Ku Klux Klan Act of 1871? Yep.

The 1871 Congress really tried to fix things only to have at least one other law illegally changed, which caused Qualified Immunity.

https://www.nytimes.com/2023/05/15/us/politics/qualified-immunity-supreme-court.html

Terrorist acts are illegal, but being associated with domestic terrorist organizations is not illegal in the United States. Affiliation with hate groups has been found by our courts to be protected under the 1st amendment. That being said any radical group not associated with white supremacist ideology is going to get be actively pursued by the US intelligence apparatus extrajudicially.

What? They have had periods of high interest and low interest. It has never been illegal to be a member lol

Being a member of the kkk being illegal in America, pfft, who would arrest you? The cop in the hood next to you? Lol