You don't "join" as such, if you're part of the class, you're part of the class and actually have to opt out or you will lose your ability to sue separately. At some point, you'll need to send in verification to get whatever the settlement is -- usually they contact you about that first, but usually there's a website to register on in case you've been missed. If you're talking about actually having a say in the lawsuit, I guess you'd have to contact the lawyers representing the class.