Ruling on one who prays alone and then another joins him

One of our brothers from al-Khafji, Nasser 'Adim al-Dawasari, our brother, says: A person entered the mosque after the congregational prayer—the 'Asr prayer—had ended and began praying alone. While he was praying, another person entered. Does he join him in the prayer, and is their prayer considered a congregational prayer?
Answer: Yes, this is the best course of action. If he stands to pray and another person comes, it is best for him to line up behind him on his right. If two people enter, they line up behind him so that they may obtain the virtue of the congregation. It is established that he (peace and blessings be upon him) saw a man who had entered after the prayer and missed it, and he (peace and blessings be upon him) said: .
The Sunnah is for some of the brothers or those present to stand and pray with the one who entered so that he may have a congregation. However, if it is one person, he lines up on his right; if it is more than one, they stand behind him. This is the Sunnah and this is what is prescribed, and in this there is great virtue; because the man obtains the virtue of the congregation by his brothers standing with him, even if it is only one person. Yes.
The presenter: May Allah reward you. So, is the one who says that the congregational prayer is only with the regular imam mistaken?
The Shaykh: No, that is a mistake and has no basis. But it is obligatory to hasten to the congregational prayer and to pray with the imam and not be late. However, if he is able to be late due to a reason, and he finds someone to pray with, the reward of the congregation is hoped for him due to the generality of the evidence. Yes.