No duty to intervene. If you want someone bound by your lawsuit, join them.
A fine point: parties are bound only by points actually litigated and determined in prior suits. This was a stipulation. That's a nice issue preclusion trick.
There does seem to be some baseline consitutional requirement, but it's not obvious what that is.
Notice + other procedures is sufficient to bind the class.
They lose as a 23(b)(2) class on commonality, and because of the back pay claim (this be the only means of recovery for former employees-- injunctive relief won't cut it-- but then again not everyone is owed back pay).
But the dissent suggests that a 23(b)(3) effort mmight succeed.