Question Summary:
What is the ruling regards congregational dua after sunnahs?

Question Detail:

What is the ruling regards congregational dua after sunnahs ?

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Congregational dua after fard or sunnah is mubāh (permissible). The general principle of a mubāh action is that one should not regard it as compulsory as this will lead to bid‘āh (innovation).
A detailed article on this is found in the following link, along with guidelines for Imams to avoid bid‘āh.
Congregational Dua after Salah


And Allah Ta’āla Knows Best
Hisham Dawood
Student Darul Iftaa
Chicago, USA 
Checked and Approved by,
Mufti Ebrahim Desai.