Is it okay for me as a Christian to join the KKK or Aryan Nation or other white-centric groups?

Yes, racism is not sin.  Bible never condemns or forbids racism, so it is okay for you to join racist organizations.  Just look through the Ten Commandments to see what is explicitly forbidden in the Moral Code of the Bible.  Read through Leviticus and Deuteronomy to find out the application of that Moral Code.  Racism is not condemned in the New Testament or the Old Testament.

You can be a great Christian and a racist.  These two are not mutually exclusive.

But you cannot be tolerant of homosexuality and call yourself a great Christian, because both the Old Testament and the New Testament calls homosexuality evil and unnatural.

The United States Constitution protects freedom of speech and freedom of association, so the US Constitution protects the rights of white Christians in the USA to join the KKK or the Aryan Nation. or other white-centric organizations.  In the same way, African-Americans are protected under the US Constitution to join black-centric organizations, such as Black Panthers, Black Lives Matther, or Black is Beautiful Movement.