Can the one who comes late to the prayer pull back the person who is standing next to the imam if he is bowing, or should he just stand next to him?
Question

                If two men are praying, and when they bow (rukoo‘), someone who came late to the prayer comes in and wants to catch up with the rak‘ah, can he pull back the one who is following the imam, or should he pray next to him until the imam says Sami‘a Allahu liman hamidah (Allah hears those who praise Him – signaling the rising from bowing), then they may both move back (to form a row behind the imam)? Or what should he do?
Praise be to Allah.
    This matter is broad in scope, in sha Allah. He may pull him back whilst he 
    is bowing, or he may stand with him, then when they rise from bowing they 
    may both move back. 
It 
    says in al-Minhaaj: The male should stand on his (the imam’s) right, 
    then if another man comes he should say the takbeer of ihram (to enter the 
    prayer) standing on the imam’s left, then the imam should step forward or 
    they should step backwards, which is preferable. End quote.

     Al-Khateeb ash-Sharbeeni (may Allah have mercy on him) said: 
    
If 
    another male comes, he should say the takbeer of ihram (to enter the prayer) 
    preferably standing on the imam’s left. Then after entering the prayer, if 
    it is possible either to step forward or step backwards, the imam should 
    step forward or they should step backwards, whether they are standing or 
    bowing, as our Shaykh said. 

    End quote from Mughni al-Muhtaaj (1/491) 

    al-Bujayrimi (may Allah have mercy on him) said: “whether they are standing” 
    or bowing, or standing again after bowing, unlike other (parts of the 
    prayer). If he is reciting the final tashahhud, then it is not Sunnah to do 
    that, because it cannot be done except with a great deal of effort and 
    trouble.

    End quote from Tuhfat al-Habeeb ‘ala Sharh al-Khateeb (2/135) 

    What we have mentioned above is by way of recommendation. If they both stand 
    beside the imam, when he is bowing or otherwise, their prayer is still 
    valid, especially if that is easier for them or is less likely to cause 
    confusion. Then when they rise from bowing, they may move to the preferred 
    place. 

    Al-Bahooti (may Allah have mercy on him) said: If they – that is, the two 
    people praying behind the Imam – stand with him – i.e., the imam – on his 
    right, or they stand on either side of him, that is valid.

    End quote from Kashshaaf al-Qinaa‘ (1/486) 

    And Allah knows best.