Slaves are another illegal way of making money as a civilian.
The slave trader is located in the far South East part of the map. Slaves cost $25,000 each. So it is recommended that you at least have $100,000 on you when you get there. This will purchase you four slaves.
To get them in your vehicle it works the same as squads in ArmA 2. You press 'Space' to select all of them, point and your vehicle and click the option 'Get In'.
Now drive down to either of the 3 Workplaces marked on the map (Go for one which has the lowest activity for cops) and park inside.
Every four to five minutes, exit your vehicle and scroll to the option where it says to collect the money that the slave has earnt you. Repeat this process for the other slaves if any.
Make sure to watch out for cops. They can arrest you and release your slaves, meaning you lose them.