Du`a' is a form of `Ibadah (worship) and all acts of `Ibadah are based on Tawqif (a religious text and not personal opinion) for no one is allowed to offer any `Ibadah that is not ordained by Allah (Exalted be He). Regarding the issue mentioned in the question, there is no authentic narration to the effect that the Prophet (peace be upon him) made a collective Du`a' with his Sahabah for any dead person after offering the Funeral Prayer. Rather, it is authentically narrated from the Prophet (peace be upon him) that he would stand by the grave after it had been leveled and say:
Seek forgiveness for your brother, and beg steadfastness for him, for he will be questioned now.
Hence, the correct view is that making Du`a' collectively and immediately after finishing the Funeral Prayer is not permissible; such practice is tantamount to Bid`ah.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.