Rule 21 Misjoinder and Nonjoinder of Parties

Adding or removing a party from a lawsuit does not require starting the case over. The court can add or drop someone from the case at any point, either because a party asked for it or because the court decided to do it on its own.

If someone was mistakenly included in a lawsuit, that alone is not a reason to throw the whole case out. The court can simply remove that person and let the rest of the case continue. The court can also split off a claim against one party into a separate case if that makes more sense.