What is the ruling on the congregants saying "Allahu Akbar" during the Friday sermon if the preacher says "Allahu Akbar" in an enthusiastic manner?

Raising the voice with takbir during the khutbah (definition) is contrary to the Sunnah (definition), as it was not customary during the Prophet's ﷺ khutbah, nor that of the righteous predecessors who followed him.

If the khatib says, "Unify Allah" (Wahhidu Allah), intending to guide people to Tawhid (definition) as a belief, not as an utterance made aloud, then there is no harm in his saying. However, the listeners erred by raising their voices. He should, therefore, guide them to his true intention.

And if the khatib intends for them to respond immediately with tahlil and takbir and by raising their voices, then he is mistaken and an innovator, and so are they; for this was not known to be practiced by the Prophet ﷺ nor by the Rightly Guided Caliphs.