Praise be to Allah.
What the believers should do is follow the example of the Prophet (blessings and peace of Allah be upon him) with regard to what he did and did not do. He should do what the Messenger (blessings and peace of Allah be upon him) did and refrain from that from which the Messenger (blessings and peace of Allah be upon him) refrained. Allah, may He be exalted, said (interpretation of the meaning):
Indeed in the Messenger of Allah (Muhammad SAW) you have a good example to follow for him who hopes in (the Meeting with) Allah and the Last Day ...
[al-Ahzaab 33:21]
Duaa by the imam after an obligatory prayer Jumuah or otherwise with the congregation saying Ameen is something that was not done by the Messenger (blessings and peace of Allah be upon him) or by any of his Companions (may Allah be pleased with them). If it was good, they would have done it before us.
Based on that, this action comes under the heading of reprehensible innovation. The Prophet (blessings and peace of Allah be upon him) said:
Whoever among you lives after I am gone will see great disputes; you must adhere to my Sunnah and the way of the Rightly Guided Caliphs. Hold on to it and cling fast to it. And beware of newly-invented matters, for every newly-invented matter is an innovation and every innovation is a going astray.
Narrated by Abu Dawood, 4607; classed as saheeh by al-Albaani in Saheeh Abi Dawood.
And the Prophet (blessings and peace of Allah be upon him) said:
Whoever introduces something into this matter of ours that is not part of it will have it rejected.
Narrated by al-Bukhaari, 2697; Muslim, 1718
Ash-Shaatibi (may Allah have mercy on him) mentioned congregational dua after the obligatory prayer in his book al-I‘tisaam (1/349-355), and explained that this is an innovation, because that was not done by the Messenger (blessings and peace of Allah be upon him) or by any of his Companions (may Allah be pleased with them) or any of the imams (leading scholars) after them.
What you must do is refrain from this innovation and strive to recite the adhkaar and duas that the Prophet (blessings and peace of Allah be upon him) used to say after the prayer. And whoever wants to offer duas may do so quietly.