To unlock Sub-Jobs you only need to have completed any character's Chapter One. After that it's a just a matter of finding corresponding Shrines scattered around the world map. The shrines appear on the radar as a curved platform supporting four pillars. In addition, there are 4 advanced jobs that can be unlocked after defeating a powerful boss inside of their Level 50 dungeon.

Unlockable	How to unlock
Apothecary	East Saintsbridge Traverse (Shrine of the Healer)
Cleric	Western Stillsnow Wilds (Shrine of the Flamebearer)
Dancer	Northern Wellspring Sands (Shrine of the Lady of Grace)
Hunter	East Victors Hollow Trail (Shrine of the Huntress)
Merchant	Moonstruck Coast (Shrine of the Trader)
Runelord	West Everhold Pass (Shrine of the Runeblade)
Scholar	Western Noblecourt Flats (Shrine of the Sage)
Sorcerer	Eask Duskbarrow Trail (Shrine of the Archmagus)
Starseer	Western Wispermill Flats (Shrine of the Starseer)
Thief	South Quarrycrest Pass (Shrine of the Prince of Thieves)
Warmaster	North Riverford Traverse (Shrine of the Warbringer)
Warrior	North Stonegard Pass (Shrine of the Thunderblade)