Ruling of praying alone behind the rows of congregational prayer
Is it permissible for a person standing behind the rows to pull back a person standing in an earlier row to pray next to him?
Praying alone behind the row
It is valid to stand alone behind a group if there is an excuse such as not finding anyone to form a row with, although in the absence of an excuse it is disliked (makruh).
Scholarly opinion for the person who does not find a gap in the row
The opinion of the Malikis and one of the opinions of the Imam al-Shafi'i, which was preferred by al-Qadi Abu al-Tayyeb, is that a person who finds himself standing alone behind the row is not to pull back anyone [to stand in the row with him] else he would deprive that person of the merit of standing in an earlier row. The Malikis added that a person standing in another row must not comply with the one who pulls him to join him in the row. This is likewise the opinion of the Hanafi scholar, Kamal ibn al Hamam.
The opinion of the Hanafis, and the preponderant opinion of the Shafi'is, is that it is recommended for the person who finds himself standing alone in a row during a group prayer to pull someone from another row to form a row with him, but only if he knows that the latter will accept. Otherwise, he is not to pull him back to avoid creating a commotion.
The opinion of Hanabli scholars is to stand to the right of the imam if possible because that is the position he would occupy if he was praying alone behind the imam. If this is not possible, he may gesture to someone from the previous row and invite him to stand next to him. Otherwise he is to stand alone behind the rows. According to Hanbali scholars, it is disliked to pull a person back; Ahmed and Ishaq, find it offensive because it entails not taking that person's permission.
The majority of scholars agree that the prayer of a person standing by himself behind the rows is valid as long as he cannot join a row. The scholars who permit pulling someone from an earlier row stipulate that he know his approval in advance. For this reason, we limit permissibility to the case outlined above; but if a person who is standing by himself behind the rows does not know whether or not he will accept or if he knows that he will not accept, then he is not to draw him out of respect and to avoid any commotion.
God Almighty knows best