A joinder is a set of documents that are filed during a divorce proceeding or immediately thereafter, making a retirement or pension plan a party to the court case. The court requires that some plans be “joined” to the case. This allows the plan administrator to become aware of the proceeding and be given an opportunity to bring any important information to the case. The joinder must be served on the plan administrator who must respond within thirty days. The joinder must be completed before a QDRO will be processed.