It is technically not a class action lawsuit right now. There is a petition for it to be classified as such but it is up to the court to grant that status.
I know that this isn't the US but it may be somewhat similar. For it to be a class action there has be a significant amount of people who have suffered that harm from that action. Also they plaintiffs would have to suffer from very similar harm and from very similar causes.
I think that they may get classification on one of their cause of action which is those that have been abused by Elders, but I really doubt that it will extend to the cause of action that the perpetrator was just a member of Jehovah's Witnesses and is not an agent of Watchtower.