Praise be to Allah.
What is best for the pilgrim on this momentous day is to strive hard in du‘aa’ and beseeching Allah, may He be glorified and exalted, raising his hands, because the Messenger (blessings and peace of Allah be upon him) strove hard in du‘aa’ and supplication on this day until the sun set. He did that after praying Zuhr and ‘Asr together in the valley of ‘Arnah, then going to the place of standing, and he remained there beside the rocks and the mountain of supplication which is called Jabal Ilaal. He strove hard in du‘aa’ and dhikr, raising his hands, facing the direction of the qiblah, atop his camel. Allah, may He be glorified, has prescribed for His slaves that they should offer du‘aa’, with humility and in private, humbling themselves before Allah, may He be glorified and exalted, with hope and fear. This is one of the best places and times for du‘aa’, because Allah, may He be exalted, says (interpretation of the meaning): “Invoke your Lord with humility and in secret” [al-A ‘raaf 7:55] and “And remember your Lord within yourself” [al-A ‘raaf 7:205]. And in as-Saheehayn it is narrated that Abu Moosa al-Ash‘ari (may Allah be pleased with him) said: The people raised their voices in supplication and the Messenger of Allah (blessings and peace of Allah be upon him) said: “O people, take it easy, for you are not calling upon One Who is deaf or absent; rather you are calling upon One Who is All-Hearing, All-Seeing. The One upon Whom you are calling is closer to one of you than the neck of his mount.” And Allah, may He be glorified and exalted, praised Zakariyya (peace be upon him) for that, as He, may He be glorified and exalted, said (interpretation of the meaning): “(This is) a mention of the mercy of your Lord to His slave Zakariya (Zachariah), When he called out his Lord (Allah) a call in secret, ” [Maryam 19:2-3]. And Allah, may He be glorified and exalted, says (interpretation of the meaning): “And your Lord said: ‘Invoke Me, (i.e. believe in My Oneness (Islamic Monotheism)) (and ask Me for anything) I will respond to your (invocation)’” [Ghaafir 40:60]. And there are many verses and hadiths that urge us to remember Allah (dhikr) and call upon Him (du‘aa’). On this occasion in particular, it is prescribed to remember Him a great deal and call upon Him, devoting that worship solely to Him with presence of mind and hope and fear. It is prescribed to raise the voice with that dhikr and du‘aa’, and with the Talbiyah, as the Prophet (blessings and peace of Allah be upon him) and his Companions (may Allah be pleased with them) did. It was narrated from him (blessings and peace of Allah be upon him) that he said concerning that day: “The best of du’aa’ is du’aa’ on the day of ‘Arafah, and the best that I and the Prophets before me said is ‘Laa ilaaha ill-Allah wahdahu la shareeka lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadeer (There is no god but Allah alone, with no partner or associate; His is the dominion, to Him be praise, and He has power over all things).”
But with regard to communal du‘aa’, I do not know of any basis for that, and to be on the safe side it is better not to do it, because it has not been narrated from the Prophet (blessings and peace of Allah be upon him) or from his Companions (may Allah be pleased with them), as far as I know. But if a person says du‘aa’ among a group of others and they say Ameen to his du‘aa’, there is nothing wrong with that, as in the case of du‘aa’ al-qunoot, or du‘aa’ upon completion of the Qur’an (khatm al-Qur’an), du‘aa’ for rain and so on.
But as for gathering on the Day of ‘Arafah, in ‘Arafah or elsewhere, there is no basis for that in any report from the Prophet (blessings and peace of Allah be upon him). The Prophet (blessings and peace of Allah be upon him) said: “Whoever does a deed that is not part of this matter of ours, it will be rejected.” Narrated by Muslim in his Saheeh. And Allah is the source of strength.
End quote from Majmoo‘ Fataawa ash-Shaykh Ibn Baaz (17/272)