Question: There are some people who do not attend the prayer except late and then they penetrate the rows until the first row becomes crowded and they have little respect for those who preceded them. So is their action permissible and what is your advice to them?
Answer: It behooves the Muslims to have concern in the rows of the prayer and completing the first row then the next and closing the gaps and spaces between them as the Prophet (ﷺ) directed to that. As for what the questioner mentioned of people penetrating the rows until the first row, then this is if there is in the first row a gap not filled. Therefore, there is no blame on these people for penetrating to it because the one in these rows did not fill the gap and they dropped their right and left off an affair obligatory upon them. So those who penetrate towards the first row and fill it are rewarded in that because they walk to a gap in the row to fill it.
If the intent is that these people enter between the congregants and there is not a gap and they only enter it to bother the congregants and create a gap that is not present, then this is not allowed. It is not allowed because in this is disturbance of the congregants and busying the congregants. And in it is stepping over the necks of the people without justification.
[Taken from: Noor ‘ala Darb Shaikh Saalih al-Fawzaan pg. 105-106]