How To Hire Secret Contribution Workers In Crimson Desert

Contribution Workers are temporary workers that can be sent on Dispatch Missions to free up your more important Comrades with much more valuable skills.
As an example, if you have a Comrade with level 3 farming and you send them on a farming mission. Instead of wasting more Comrades, you can send the Contribution Workers. You still get the Comrade Bonus and Skill Bonus, but all those Comrades are now free to do other things.
The item you need is called the Seal of Devotion. It is a reward for completing the Letter of Silence Faction Quest for House Azerian, under the Flame Knights quest chain.
Once you defeat Tristan you will be rewarded with the Seal of Devotion. You can then use this item from your inventory to unlock hidden workers.

To use Contribution Workers simply select any Dispatch Mission, and the workers in the regions where you have Contribution points will be displayed.