Is it permissible for a group to supplicate together upon the completion of reciting the Qur’an?

It is permissible for a Muslim to gather his family and companions upon the completion of the Qur'an to supplicate, and for them to say "Amen" to his supplication. Al-Qurtubi reported that the Salaf (early Muslims) used to do this. Ad-Darimi and Al-Bayhaqi narrated that Anas ibn Malik used to gather his children and family upon completion (of the Qur'an) and supplicate for them. As for everyone supplicating in one voice and in unison, this is an innovation (bid'ah) with no basis in Islamic law.