This is how to unlock people during the game.

Mia-Talks with Ike during Chapter 7

Ilyana-Talks with Ike during Chapter 8

Marcia-Talks with Ike during Chapter 9

Lethe and Mordecai-Will join only if they survive Chapter 9

Volke-Will join if you hire him during Chapter 10

Nephenee and Brom-Open their cells and talk to them with Ike during Chapter 10

Kieran-Unlock his cell and talk to him with Oscar during Chapter 10

Zihark-Talk with either Mordecai or Lethe during Chapter 11

Jill-Talks with Ike during Chapter 12

Sothe-Go into "Info" before Chapter 12 and select the 3 stared report and allow him to join you

Astrid-Talks with Ike during Chapter 13

Gatrie-Talks with Astrid during Chapter 13

Makalov-Talks with Marcia during Chapter 14

Stefan-During Chapter 15, move either Lethe or Mordecai into the upper right hand corner of the map

Tormod-Joins when you beat Chapter 15

Maurim-Joins when you beat Chapter 15

Devdan-Talks to Soren during Chapter 16

Reyson-Beat Chapter 17, then before Chapter 18, allow him to join your team

Calill-Read the 3 Star "Info" and allow her to join you before Chapter 20

Tauroneo-Talk with Ike, Soren, Mist, Tormod or Sothe during Chapter 21

Haar- Talk to Jill during Chapter 23

Geoffrey-Will join you only if he survives Chapter 24

Largo-Read the 3 Star "Info" before Chapter 25

Ena-Joins your Team if the Black Knight is not defeated during Chapter 28

Nasir-Joins your team if the Black Knight is defeated during Chapter 28