Sheikh `Abd al-Rahmân al-Barrâk
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 anlawful innovation (bid`ah). The burden of proof, therefore, is upon the one claiming that some act is an act of worship.
There is no authentic hadîth in existence that claims that we are supposed to perform dhikr in unison. Therefore, it is an innovation to do so.
None of the hadîth you refer to indicate in any way that the Companions engaged in dhikr congregationally. They simply say that the Companions used to remember Allah in groups.
This does not imply that they did so in concert with each other and with one voice. For example, Muslims in the mosque are together engaged in the remembrance of Allah, while in fact some of them are reading Qur’ân. Some may be engaged in teaching or learning the Qur’ân. Others may be praying while some may be praising Allah and remembering him using the forms of remembrance taught by the Prophet (peace be upon him).
Some may be holding study sessions wherein they are indeed participating together in the remembrance of Allah, but not by chanting in unison.
Source: Islam Today