Is there additional sawaab for performing nafl and nazr Salaah with Jamaat?

With regards to nafl Salah, there will be additional sawaab in performing it in jamaat provided there are 4 people in total (i.e. including the Imaam). If there are more than 4 people, it will not be permissible.

As far as Nazr Salaah is concerned, it is not permissible for people to perform it in Jamaat since each person’s nazr is different, however in the case where they all have taken the same nazr it will be permissible to perform it in Jamaat and more rewarding than performing it individually (e.g. Few sons of a sick man had made nazr to perform nafl Salaah if Allah blesses their father with shifaa. If their father is blessed with shifa, they all can complete their nazr of performing Salaah in jamaat.)

And Allah Ta'ala (الله تعالى) knows best.