sororities at university - do you have to join one in the USA? Can a woman choose to not take part ?

Here in Australia it's not really a thing but it seems to entrenched into USA university culture. I guess for guys the fraternity thing is basically the same but men instead of women.

Been reading up on that Bama Rush program and sooooo glad I wasn't going to uni in the US.

I still don't fully get the purpose behind the 'greek system collegiate residence' concept.