A person may be deemed legally represented by another party without their direct participation in four narrow circumstances: statutory schemes authorizing representation, formal mechanisms like class actions where representatives are named, fiduciary relationships where one party has a legal obligation to act on behalf of another, or representation by an existing party with the same interests and adequate representation.
For example: A shareholder might be bound by judgments in derivative suits brought on behalf of the corporation by other shareholders because their interests are aligned and presumed to be adequately represented by the plaintiffs in such lawsuits.