when your I-485 is approved (or you know you will be approved in a couple of months) you can file one I-824 to start all your derivatives' following to join process. This type of I-824 will need your 485 approval notice at the time of ajudication, so don't file it too early. Once I-824 approved, NVC will start communicating with you for fees and documents. After this is cleared, the case will be forwarded to GZ consulate, and currently a contractor is handling the scheduling that used to be done by consulate itself. You need to register with the contractor.