DUA COLLECTIVELY AFTER NAMAAZ
Al-jawab billahi at-taufeeq (the answer with Allah's guidance)
To make Du'a individually after the various Fard prayers is an act of Sunnah. This was the practice of the blessed Messenger of Allah (Sallallahu Alaihi Wasallam), his Companions (RA) and the pious predecessors (RA).
Mughira ibn Shu’ba (Allah be pleased with him) narrates that the Messenger of Allah (Allah bless him & give him peace), used to say after completing his prayer: “There is no god but Allah, alone without partner. His is the dominion, His is the praise, and He has power over all things. O Allah, none can withhold what You bestow, none can bestow what You withhold, and the fortune of the fortune avails nothing against You” (Sahih al-Bukhari, 11/133 & Sahih Muslim, 5/90).
To make Du'a collectively is permissible. However, It will not be permissible to project a permissible act as compulsory as this is projecting a wrong view of the Shari'ah, thus, causing distortion of Deen. To regard the collective Du'a after Fardh Salaat etc, as compulsory will be Bid'ah as that projects the incorrect image of Shari'ah.
Also, the following should be avoided when making Du'a collectively:
1) Making a permissible act as compulsory.
2) Following the Imam in the collective Du'a and making it compulsory.
3) Imam raises his voice in the Du'a and making it compulsory. It is peferable to make Du'a in a low tone.
Allah Ta'ala says,
“Call on your lord with humility and in private” (al-A’raf, 55).
4) Making the collective Du'a in a raised voice and causing interruption to the Masbooq (a person who has missed a Rakat).
5) It is Makruh (disliked) Making the collective Du'a longer than 'Allahumma Antas Salamu.......' in those obligatory prayers that has the Sunnats after it. (Raddul Muhtar Vol. 1 Pg. 494)
6) Giving more importance to the collective Du'a than any Faraidh (obligatory) acts.
And Only Allah Ta'ala Knows Best.
Collective dua of the Imam and Muqtadi (follower) after Salah: There is no evidence in the Sunnah for it and since it is disruptive to late-comers of Salah (who are trying to complete their Salah) & in our times people have started considering it compulsory and a part of Salah; it should be be avoided. Mufti Ibraheem Desai (DB) advises:
The ulama should first educate their congregation on the sunnah method of Adhkar after salah and gradually inform them that the congregational dua is not compulsory and was not practised at the time of Rasulullah (Sallaho Alaihe Wassallam).Get the congregation to first understand that congregational dua is mubah and not compulsory. Do not condemn the practice without educating the congregation. This will lead to fitna.