All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The meaning of the saying of the Prophet : “…the first of them will not enter till the last of them would have entered.” is that all of them will enter at the same time in one row and in one go, but it does not mean that other people than them would not enter. The narration by Al-Bukhari reads: “holding on to one another.”
Ibn Hajar said in “Fat’h Al-Baari”: “It means that they will enter in one row, so all of them will enter in one go, and describing them by the first and the last is according to the time at which each of them has crossed the Siraat Bridge, and this indicates that the door through which they will enter Paradise is so vast.” Besides, Iyaadh said: “The meaning that they hold on to one another refers to sedateness so no one would try to compete with the other when entering; rather, they will all enter together (in one go).”
An-Nawawi said: “It means that they will enter in one row (horizontally) one beside the other.”
Allaah Knows best.