Scholars hold different views regarding praying individually behind the row. However, a Muslim should be keen not to do so without a valid reason.

In his well-known book, Fiqh As-Sunnah, the late Sheikh Sayyed Sabiq states: “According to the majority of scholars, if a person prays individually behind the rows, his salah (Prayer) will be valid, though it is disliked doing so.
Ahmad, Ishaq, Ahmad, Ibn Abu Laylah, Waki`, Al-Hassan Ibn Salih, An-Nakha`i and Ibn Al-Mundhir hold the view that if a person prays one complete rak`ah (Prayer unit) behind the rows, his salah will be invalid.
Wabsah related that the Messenger of Allah (peace and blessings be upon him) saw a man praying individually behind the rows, and the Prophet ordered him to repeat his salah.
`Ali Ibn Shayban related that the Messenger of Allah (peace and blessings be upon him) saw a man praying behind the row and he waited for him (until he finished the prayer and) told him: “Go forward (and join the row) for the salah of a person standing alone behind the rows is not valid.” (Reported by Ahmad)
The majority of scholars stick to the hadith of Abu Bakra who said that he preformed part of the Prayer, behind the row, and the Prophet did not order him to repeat his salah. Thus, repeating the salah signifies an over-zeal in practicing what is recommended and better.
Al-Kaman Ibn Al-Hamam said: “Our scholars are of the opinion that the hadith of Wabsah refers to what is preferred while the hadith of `Ali Ibn Ash-Shayban underlines failure to practice what is the best and, as such, they are in harmony with the hadith of Abu Bakra. It is clear, then, that it is not necessary to repeat the salah because such an act was not always ordered.”
If someone comes to the row and does not find sufficient space or a gap to stand in the row, then, according to some scholars, he should stand individually and it is disliked that he should pull anyone back from the row. Others say that he should pull one, who is aware of this ruling of Shari`ah, back from the row after they have performed the opening takbir, and it is preferred for the one who is pulled to join him.”