Is the Du'a in group after Salah Faridhah or after Taraweeh acceptable or it is Bid'aah, many people comes to the Imam in our local mosque in north London and ask him to make Du'a for them after Taraweeh specially and all raises their hands except me and few people.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. Du'a in the Witr of Taraweeh prayer is not an innovation. It is approved by Shariah. In this situation, the Imam says the Du'a and the people praying with him say Ameen as a follow-up to his invocations. Similarly it is lawful to make Du'a after the obligatory prayers but only individually so that the Imam supplicates for himself and each person praying behind him makes Du'a for himself individually. But invoking in groups in Fard (obligatory) prayers is an innovated act which is not reported to have been done by the Prophet (Sallallahu Alaihi wa Sallam) or his companions. What you described in your question is a form of this innovation, above all, if these people make such group Du'a a habit. We remind the questioner that the Du'a of the Muslim for his/her Muslim fellow is not only lawful but also desirable in Shariah. Allah knows best.