If He did not find a Place in the First Row
Question:
What is the ruling if a worshipper entered the mosque and he did not find a place in the first row? Is it permissible for him to pull a person from the first row, or what should he do?
Answer:
If a man entered the mosque and found the row to be completed and could not find any place in the row, then he must wait until he finds a place or someone arrives to pray with him, or, he may stand to the right of the Imam. He may not pull a person out of the row, because the Hadith which states this is weak, and also because removing a person from the row would cause a gap therein, and the Prophet (sallallaahu alaihi wa sallam) ordered us to close up the rows. And through Allaah is success attained.
Source:
Shaykh `Abdul-`Azeez Bin Baz
Fatawa Islamiyah Vol. 2 Page 164