Straightening the saffs

Pulling a person back from the front saff in Jamaat Salaah

Q: In Salaah in Jamaat, when pulling a person back from the front saff to join him in the back saff so he doesn't read Salaah alone in the saff: 

  1. Does he pull the person from the middle of the saff or from the side of the saff?
  2. How will the front saff get completed again? 
  3. If a third person then came and joined the second saff, can any one of them move into the front saff if that space is still open?