The evidence is that no prohibition of this has been reported. Something similar occasionally occurred among the Companions: they would ask the Messenger, peace and blessings be upon him, to supplicate for them. But in this particular setting, I do not know of any report that he led the people in supplication. Therefore, we say that it is preferable for each person to supplicate for himself, especially if he fears that supplicating with his companions may open the door to collective talbiyah and the like.
“Whoever stands [at ‘Arafah], even for a moment, from the dawn of the Day of ‘Arafah until the dawn of the Day of Sacrifice, while being eligible for it, his Hajj is valid; otherwise, it is not, ………
His statement, “Whoever stands”: the word “whoever” is a conditional noun and thus includes everyone who has entered into the state of ihram for Hajj. Therefore, if someone stands at ‘Arafah but does not enter ihram until after leaving it, his standing there does not benefit him.
His statement, “even for a moment”: this may indicate a point of juristic disagreement, or it may be for emphasis—that even if he stands for the briefest period, it suffices. This is the more likely meaning.
His statement, “from the dawn of the Day of ‘Arafah until the dawn of the Day of Sacrifice”: the author, may Allah have mercy on him, informs us that the time for standing begins at dawn on the Day of ‘Arafah. This is one of the distinctive positions of Imam Ahmad’s school. The majority of scholars hold that the time for standing begins only after the sun has passed its zenith; this is also one narration from Imam Ahmad.
The proof of Imam Ahmad’s well-known position, may Allah have mercy on him, is the hadith of ‘Urwah ibn Mudarris, may Allah be pleased with him. He came to the Messenger of Allah, peace and blessings be upon him, at Muzdalifah for the dawn prayer and told him what he had done: that he had exhausted himself and worn out his mount, not leaving any mountain without standing upon it. The Prophet, peace and blessings be upon him, said to him: “Whoever attends this prayer of ours and stands with us until we depart, having stood at ‘Arafah before that by night or by day, has completed his Hajj and fulfilled his rites.” (1)