What is the ruling on communal remembrance (dhikr) after the Friday prayer in the described form, and how should one advise those who say that it is a legitimate form of remembrance and a way of teaching people, and that the Prophet's hadiths did not specify whether remembrance should be secret or public?

Remembrance (dhikr) is مطلوب (enjoined) and مشروع (prescribed) at all times, as Allah the Almighty says: [Remember Allah with much remembrance] and […men who remember Allah much and women who remember Him much] and […who remember Allah while standing, sitting, and on their sides]. And the Prophet, peace and blessings be upon him, said: "Let your tongue be moist with the remembrance of Allah." However, specifying a certain remembrance (dhikr) to a particular time or a specific number requires evidence, otherwise it is an innovated addition (bid'ah idafiyyah), due to the Prophet's saying, peace and blessings be upon him: "Whoever performs a deed that is not in accordance with our matter, it will be rejected." The method of dhikr employed by this preacher is an innovation (bid'ah) that was not reported from the Prophet, peace and blessings be upon him, nor from the Tabi'un (successors). You should try to convince the preacher gently and present to him the sayings of the people of knowledge (scholars) away from the public. If he does not accept, then you have discharged your responsibility, and those worshipers who can be advised should be advised.