Legal Question in Civil Litigation in California

You have two choices. You can persue recovery from the retirement administrator for their error, which they may or may not fight. If they lose, they will then seek to recover from the other beneficiaries. The other alternative is to pursue recovery directly from the other beneficiaries, but if the money is gone that would do little good. If you find you need to file suit, it would probably be best to name them all and let them sort it out. You haven't mentioned how much money is involved, so I cannot say if you have the alternative of using Small Claims Court or not. If you find you need to go after the administrator, and it is more than the $7,500 Small Claims jurisdiction, you will need an attorney, because the plan will certainly have a team of them to throw at you.