You would have to get an order allowing you to amend the complaint. If you cannot get the other parties to stipulate, you have to bring a noticed motion for leave.
You also have to make sure that the statute of limitations has not barred an action against the other partner. The statute of limitations continues to run until you amend the complaint. It does not stop when the action was first brought.
If the other cross-defendants have already answered, then you need permission from the court to file an amended pleading. A stipulation filed with this motion from the other parties agreeing to this will help ensure that the court will grant that motion.
Hire a new lawyer if you can. Maybe there was a reason the lawyer didn't add this partner.
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Did the cross complaint have any Doe cross defendants? Are there charging allegations against the does? Has the statute of limitations run? Id consult with a new atty asap to discuss your options re adding the partner. There could be reasons not to, and it could be as simple as serving him as a Doe deft.
I agree with all of the information Ms. Koslyn has provided. Just to add one more bit of procedural information--if you do decide to file an amended pleading, do not forget to also serve a copy of said pleading on the cross-Δs “affected thereby.”
You have already received information that is meaningful to anyone trained in the law. You also were told to get another lawyer. That is the essential point you must evaluate. You cannot solve a partnership dispute alone and certainly cannot figure out civil procedure on your own. You must have a competent lawyer. If you believe your lawyer was not providing sufficient attention to your situation or does not know enough about it, you must find another lawyer.
The advice provided is in good faith but not a guaranty of accuracy under all circumstances.