A proper party is a person or entity, such as a business, that has an interest in a case that is being heard in court in terms of bearing a beneficial or negative outcome on the judgment in the case. In those instances, a proper party can be joined as a plaintiff in a lawsuit. In the same way, another party can also be brought in as a defendant to the action. Each party would be judged together, but their sentences would be varied depending on their level of interaction or culpability in the crux of the case.
A proper party can give rise to a class action suit where several plaintiffs sue the same person or entity for damages relating to the same thing, such as a detrimental effect from a pharmaceutical medicine or negligence in a consumer product that gave rise to multiple instances of injury.