In case you haven't heard the term 'slugging,' the term refers to casual carpooling. No money exchange. The purpose is to use the I-95 HOV lane, so they get to DC faster. The slugging business even have its own website, slug-lines.com, where you can find information that you can think of about slugging there.
Though, these folks are total strangers, slugging is safe, according to Slug-Lines.
However, slugging is a very organized system with its own set of rules, proper etiquette, and specific pickup and drop-off locations. It has thousands of vehicles at its disposal, moves thousands of commuters daily, and the best part, it’s FREE!
If there's no money exchange between the driver and the riders over in DC, in the other side of the world in place like Jakarta, Indonesia, the sluggers (acting as additional passengers) are 'actually' get paid to ride in the car. Since cost of labor is cheap. You'd see people lined up along busy streets leading up to the HOV-3 area, waiting to be picked up for a ride and make some money. So, slugging is use as a way to create jobs over there.
This clip below found explains about slugging in DC.