When it comes to acts of worship, they have to be clearly and precisely established by the Qur’ân and Sunnah. Anything else is an unlawful innovation (bid`ah). The burden of proof, therefore, is upon the one claiming that some act is an act of worship.
Therefore reading Quran in unison in a gathering or sending salawat on the prophet peace be upon him in unison and in a gathering is nowhere found in the books hadith.
The proofs the previous reply presented are weak and already established at specific times and specific places. Which he cannot then draw a deduction for unison thikr at other times and places in unison. Yes, there are various ahadith which are found that state reward of making thikr in gatherings, such as, narrated by Muslim (2701)
From Mu‘aawiyah ibn Abi Sufyaan (may Allah be pleased with him), according to which the Messenger of Allaah (blessings and peace of Allah be upon him) came out to a circle of his companions and said: “Why are you sitting here?” They said: We are sitting to remember Allaah and praise Him for having guided us to Islam and blessed us with it. He said: “By Allaah, are you only sitting for that purpose? They said: By Allaah, we are only sitting for that purpose. He said: “I did not ask you to swear because I am accusing you, but Jibreel came to me and told me that Allaah was boasting of you to the angels.”
And the report also narrated by Muslim (2699) from Abu Hurayrah (may Allah be pleased with him) who said:
The Messenger of Allah (blessings and peace of Allah be upon him) said: “No people gather in one of the houses of Allaah, reciting the Book of Allaah and studying it together, but tranquillity will descend upon them, mercy will overshadow them, the angels will surround them and Allaah will mention them to those who are with Him.”
– There is nothing in these reports to suggest that dhikr should be recited in unison. But they can speak about the favors of Allah upon them, discuss business according to sunnah or remember the greatness of Allah. Which is all considered thikr and in a group.
The point is these ahadith and the implementation of them was already there during the time of prophet pbuh and his companions. And none of them understood them as unison thikr or unison Quran recitation with a single voice, as the brother before this answered. Which if it was, they would have acted upon it before us. This implies there was a shortcoming in the understanding the companions regarding our beloved prophet pbuh sayings.Which we now understand. Which is impossibble. Our best recouse is to refer all matters back to Allah and his messenger.
As Allah says:-
"If you differ in anything among yourselves, refer it to Allah and His Messenger, if you believe in Allah and the Last Day. That is best and most commendable in the end." [Al-Qur'an 4:59].
He also says:-
"And whatever you disagree upon, its decision is with Allah." [Al-Qur'an 42:10].
And whoso opposeth the messenger after the guidance (of Allah) hath been manifested unto him, and followeth other than the believer's ( sahaba) way, We appoint for him that unto which he himself hath turned, and expose him unto hell - a hapless journey's end!
(Nisaa verse 115)
So fear Allah with regards your religion and who you take it from.
The best of guidance is the book Allah and the best actions is the that of the prophet pbuh.
And Allah knows best.