Slugging is fairly unique to DC though.
There's enough people going to specific locations at specific times that make it work well. For example, I worked near the Capitol Building, and lived in Prince William County. During rush hour it could take over an hour to get from downtown to PWC using regular I-95. But by picking up a couple of people, I could then take the HOV lanes on I-95 and cut that time in half, or even less. I drop the riders off at the park-and-ride location, and then go home.
Another reason it works is because the people you are picking up are like you: workers. It's not like there's any criminals that want to go from downtown DC to the suburbs at rush hour. It's just people who want to get home after a long day of work, and not spend another hour plus doing it.
There's no compensation between the drivers and the riders. Both get benefits -- the driver gets to use the HOV lanes thanks to the riders, and the riders get a free ride thanks to the driver. Personally, I had a lot of respect for riders simply because they did save me so much time. And there were times when there were far more drivers than riders, and a driver may have to wait 10-15 minutes just to get riders.