There also rulings in Fatawa Islamiyah from the Permanent Committee stating that group du'a' is an innovation because it has not been transmitted from the Prophet

, nor from his Companions (radhiyAllaahu anhumaa'). I think it maybe posted on the site.
Habib ibn Maslama al-Fihri (Allah be pleased with him) narrates that I heard the Messenger of Allah (Allah bless him & give him peace) say: “No group of believers assemble, one of them supplicating while others saying Amîn, except that Allah answers their prayer.” (Recorded by Tabrani in al-Mu’jum al-Kabir, 4/26 & Hakim in his al-Mustadrak, 3/347 and he classed it as authentic (sahih)
Hadhrat Qais Madani narrates that when a man came to ask Hadhrat Zaid bin
Thaabit (rd) something, he said to the man, "Go to Hadhrat Abu Hurayrah
(rd) because it once happened that Abu Hurayrah (rd) someone else and I
were in the Masjid engaging in Dhikr and making du'aa when Rasulullaah
came to sit with us. When we fell silent, Rasulullaah (sw)asked us to continue
doing what we had been doing. My companion and I then started making du'aa before Abu Hurayrah (rd)- and Rasulullaah said 'Aameen' to our du'aas.
Abu Hurayrah (rd) then started making du'aa and said, ' 0 Allaah! I ask You for everything that my two companions have asked for as well as knowledge that I shall never forget.' When Rasulullaah (sw) said Ameen, my companion and I said, ' 0 Rasulullaah (sw) We also ask for knowledge that we shall never forget.' Rasulullaah said, 'The man of the Daus tribe (Hadhrat Abu Hurayrah (rd)has beaten you to it."'(Tabraani in his Awsat, as quoted in Majma uZawaa'id (V01.9 Pg.32 I)
Hadhrat Jaami bin Shaddaad narrates from a relative that he once heard Hadhrat Umar say,
"I want you people to say 'Aameen' to three du'aas that I am going to make." He then proceeded to make du'aa saying, " 0 Allaah! I am weak so please strengthen me. 0 Allaah! I am stern, so please soften me. 0 Allaah! I am miserly so please make me generous." (Ibn Sa'd (Vo1.3 Pg.275)
Hadhrat Saa'ib bin Yazeed reports that it was early one morning during the Year of Ashes (the year, in which Madinah experienced a crippling drought) that he saw Hadhrat Umar (rd) humbling himself before Allaah, wearing 'simple clothing and a shawl that barely reached his knees. He was seeking Allaah's forgiveness in a loud voice as his tears flowed on to his cheeks. (Rasulullaah' s uncle) Hadhrat Abbaas bin Abdul Muttalib (rd) was standing on his right side as he faced the Qibla and raised his hands as he sobbed before his Rabb. The people also made du'aa as he made du'aa, after which he took Hadhrat Abbaas hand and said, "0 Allaah! Do accept the intercession of Rasulullaah (sw) uncle on our behalf." Hadhrat Abaas (rd) then stood beside Hadhrat Umar (rd) for a very long time, his eyes flowing with tears as he made du'aa to Allaah. Ibn Sa'd (Vo1.3 Pg.321).
Hadhrat Abu Sa'eed who was the freed slave of Hadhrat Usayd reports, "Hadhrat Umar (rd) used to patrol the Masjid at night and remove from there everyone besides the person engaged in'salaah. When he once passed by a group of Sahabah (rd) amongst whom was Hadhrat Ubay bin Ka'b (rd), he asked, 'Who are you men?' Hadhrat Ubay (rd)replied, 'We are members of your family, 0 Ameerul Mu'mineen.' 'What kept you behind after the salaah?' Hadhrat Umar (rd) asked. When they informed him that they had been engaged in Dhikr, Hadhrat Umar (rd) Sat with them. He then said to the person closest to him, 'Take (the lead in making du'aa).' The man started making du'aa and (when he had completed) hadhrat Umar (rd)asked each person to make du'aa until he came to me, who was sitting next to him. 'Come on,' he said to me. I was tongue-tied and started to shiver so much until he could actually feel me shake. He then said, '(Say something) Even if you have to only say, ' 0 Allaah forgive me. 0 Allaah! Have mercy on me.' Hadhrat Umar(rd) then started making du'aa and there was none who wept more than him. Thereafter, he said, 'That is enough. You may all disperse."' Ibn Sa'd (Vo1.3 Pg.294).
Hadhrat Abu Hurayrah (rd) narrates that Hadhrat Habeeb bin Maslamah Fahri(rd)was a person whose du'aas were always accepted. When he was once appointed commander of an army and after making the necessary preparations, he was facing the Roman army when he said to the others, "I have heard
Rasulullaah (sw) say, 'When a group assembles and they all say 'Aameen' as one of them makes du'aa, Allaah certainly acts the du'aa."' He then duly praised Allaah and said, " 0 Allaah! Protect our blood and still grant us the reward of martyrs." He was still making du'aa when the commander of the Roman army entered Hadhrat Habeeb (rd) tent (to surrender).Tabraani. Haythami (Vol. 10 Pg. 170) has commented on the chain of narrators.
in anohter story which hadhrat Nu'maan bin Muqarrin (sw) said, "I am to make a du'aa to Allaah which I stress that every person say 'Aameen' to. (He then made the du'aa saying) 0 Allaah! Grant Nu'maan martyrdom today with your assistance to the Muslims and make them victorious." Another narration adds that the others then said 'Aameen' to the du'aa.(Tabari, reporting from reliable sources as confirmed by Haythami Vol.1. Pg.216). Haakim (Vo1.3 Pg.293) has reported a similar narration.
Hadhrat Uqba bin Aamir states, "It was with regard to a person called 1
Abdullaah Dhul Bijaadayn that Rasulullaah (sw) said, 'Indeed, he is one who sighs very deeply.' This Rasulullaah (sw) said because Abdullaah Dhul Bijaadayn used to engage in Dhikr abundantly by reciting the Qur'aan and
making du'aa and all this he did in an audible voice." (Ahmad and Tabraani, reporting from reliable sources as confirmed by Haythami (Vo1.9 Pg.369). Ibn
Jareer has reported a similar narration, as quoted in the Tafseerof Ibn Katheer (Vo1.2 Pg.395).