This act is not permissible and is a Bid`ah (innovation in religion), because
congregational Du`a' after performing Salah is a Bid`ah that has no origin in Shari`ah (Islamic law). The basic rule states that the acts of worship are Tawqify (bound by a religious text and not amenable to personal opinion). Accordingly, the wording of the said Du`a' is an innovated Bid`ah. All good lies in what was reported from the Prophet (peace be upon him) and which he has ordered his Ummah (nation) to follow, such as the wording mentioned in the second part of the Tashahhud (recitation in the sitting position in the second/last unit of Prayer) and other authentic wordings as well.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.