Is being a member of Anonymous illegal?

The KKK and NeoNazis exist in America still and are protected by the law to do so. So no its not illegal. Its just when you push the barrier and break the law when it becomes illegal. You can support them all you want and join them.

I wouldn't consider it illegal (in America at least). America has freedom of speech, so you can run around in a parking lot saying something stupid (out of context) like, "I'm your father" repeatedly, and people would just think you're stupid or mentally challenged, but the police wouldn't arrest you for something like that.

I'm not sure about UK's law, but I'm sure there's something just like it.

I don't think it's illegal to go round saying you support them, put the mask up as your avatar etc (not in the UK anyway - and lets face it, you could just say you like the film V for Vendetta...) but if you start joining in with their DDOS attacks then there are a few laws you could fall foul of here, although you'd have to be pretty unlucky to be done for it

conceptually speaking, there is no way to join anon as it is not a group. It functions so well because no one knows anyone else and there is no chain of command, and no one can snitch on anyone else.

realistically speaking, Anonymous is a group named Anonymous (kinda like how the company sunbeam is not an actual sunbeam, but instead a random name). A group that can release a press statement about a planned course of action is not truly anonymous in terms of the definition.

Following the group "Anonymous" definition of who they are is the equivalent of 1 person saying

we the 8 billion people of earth agree that 1+1=3