50% terrace share allotted to owner being claimed for usage by the owners of flats sold by builder.

My father who was owning 60x40 plot in Bangalore, entered in to joint development with a builder. In the Joint Development Agreement (with is registered) it is clearly mentioned that the owner and the builder will have a 50-50 share in number of flats to be constructed as well as in parking and terrace area. The builder has constructed 10 flats, and sold 5 flats to different persons from his share. My father got 5 flats. The builder has constructed a small office room for his usage on the terrace area, has fenced and locked the 50% terrace area. The rest 50% terrace area owned by my father is empty and is being used by us. Now the flat owners of the builders share are claiming / asking for usage rights of the terrace stating that we are occupying the terrace illegally. My question is whether the flat owners of builders share - have rights to terrace area of the owners share or the builders share. Since the registered Joint Development Agreement clearly says that owner will have 50% share in both parking and terrace areas is it not illegal on part of the flat owners of builders share to claim usage rights of the owners area?