With regards to saying Dhikr aloud and in congregation, there are many verses of the Qur'aan and Ahaadith to establish its permissibility. Suffice to mention just a few, Allah Ta'ala says, 'O you who believe! Remember Allah Ta'ala abundantly.' (Surah Ahzaab). 'Remember me and I will remember you.' (Baqarah). 'Certainly, the rememberance of Allah Ta'ala is the greatest.' (Ankaboot)

As far as Ahaadith are concerned, Nabi (Sallallaahu Alayhi Wasallam) is reported to have said, 'Allah Ta'ala says, O my servant, you remember alone, I will remember you alone. And if you remember me in a gathering, I remember you in a gathering more better.' (Tirmidhi).

Another narration mentions that 'an angel announces after the gathering which was busy in the rememberance of Allah Ta'ala that your sins have been pardoned.' (Musnad Ahmad).

Ibn Abbaas (Radhiallaahu Anhu) narrates that loud Dhikr after Fardh Salaat was the practice during the time of Nabi (Sallallaahu Alayhi Wasallam).

Regarding Reciting Dua's Aloud in Congregation
There are many Du'aas that are established from Ahaadith to be recited after the Fardh Salaat. However, to recite these specific Du'aas aloud in congregation is not permissible unless the Imaam of a Musjid does this with the congregation to teach them these Du'aas. However, when these Du'aas are learnt, this practice must stop. (Ahsanul Fataawa vol.3 pg.67)

Bid'at
Any action that was not prevalent during the time of Nabi (Sallallaahu Alayhi Wasallam) and Sahaaba (Radhiallaahu Anhum) will be termed as Bid'at. However, the fact that it is Bid'at does not necessarily render it forbidden not not permissible. There are various such examples. This type of Dhikr is also one such example. The special types of Dhikr conducted by certain Ulama are not exactly and directly proven from Qur'aan or Hadith not were they prevalent during the time of Nabi (Sallallaahu Alayhi Wasallam). These types of Dhikr are adopted as a spiritual remedy for spiritual upliftment. The constant rendition of certain Dhikrs like 'Allaahu' is for the very same reason. Since this although it is an innovation, however, it enables one to gain spiritual upliftment and in doing so it is not in conflict with any other law of Shari'ah, hence, permissible.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai, www.ask-imam.com
FATWA DEPT.