A brother told me that it is not allowed and I am actually allowed to pull someone out from the first saff to join me in the second saff. Jazakallah!
Answer:
If the first Saff becomes full, thereafter if a person is alone in the
second Saff, he should wait till another person joins him. If noone comes
then before the Imaam makes Ruku, one should indicate to a person from the
first Saff to join him by touching him. The person should be aware of such a
Mas-alah. If the person is not aware of this Mas-ala, the late comer should
stand alone in the second Saff. (Fataawa Mahmoodiya vol.1 pg.222; Mazhari)
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai