Defeat the Bosses from each base. You can then defeat them again and have the chance to recruit them at the indicated guild.

Unlockable	How to unlock
Hannah	Go to Balterossa then to the Union Of The Golden Chalice to recruit her.
Hinnah	Go to Nagapur then to the Union Of The Golden Chalice to recruit her.
Ludope	Go to Melphina then to the Union Of The Golden Chalice to recruit him.
Milton	Go to Royotia then to the Union Of The Golden Chalice to recruit him.
Snievan	Go to Elysion then to the Ring Of The Labyrinth to recruit him.
Young	Go to Baaluk then to the Union Of The Golden Chalice to recruit him.
Zuido	Go to Ghor then to the Ring Of The Labyrinth to recruit him.