Q. What does a person do if Salaah has commenced and the Saffs in Salaah are full? Does a person stand alone in the next Saff and perform Salaah?
A. In the case of the rows (Saffs) in Salaah being full, it is disliked (Makrooh) for one to stand alone in a row and join the Salaah. One should wait for another person to join the Salaah before the Imaam goes into the bowing (Ruku) posture and then join the Salaah together.
If no other person is joining the Salaah and the Imaam is going into the bowing posture, one should indicate to anyone of the followers (Muqtadis) performing Salaah from the row in front to join one in the row behind. If this is not possible or if there no follower who would understand the circumstance and procedure in such a case, one would be excused and hence, one should stand alone in the row and join the Salaah. (Shaami 1/568-Ahsanul Fataawa 10/284)
Allah Ta’ala Knows Best
Mufti Ismaeel Bassa