Some scholars say that the congregational dua after Fardh Salaat is bid'ah.

Answer

The dua after Fardh Salaat is a Sunnah practice. While it has a congregational form, it is in reality dua made individually. It is for this reason that it is Sunnat to make the dua silently. Everyone should engage in his own dua. One may begin with the Imaam, before the Imaam or after the Imaam has commenced with the dua. It is not necessary to begin and end with the Imaam. Making the dua loudly is not permissible.