Free the slaves in the slave pen, for the Rangers

 is a Fallout 2 quest obtained from Elise, the head of the Rangers club in NCR. By doing this quest you're aligning yourself with the Rangers and, respectively, you can not do it if you have the Slaver Reputation obtained from Metzger.

Walkthrough
After agreeing to become affiliated with the Rangers, Elise will direct you to free the slaves in the Bazaar area of NCR. The area is guarded by a gang of slavers so violence will be an inevitable necessity for the completion of this quest.

To free the slaves, you must use the terminals on each of the pens. Sometimes the terminals won't respond on the first interaction, so keep trying.

After returning to Elise, you will receive 3000xp, a Ranger Pin and be officially regarded as a ranger. Unfortunately, this does not change any of the game mechanics or give any unique quest possibilities further in the game, except in Redding where the sherif will recognize you as a Ranger, but this won't help you a lot.

Alternate Route
There is also a way to free the slaves without resorting to violence, though this requires exploiting the game mechanics in combat. You can flee from a combat by just walking to another area as you know, same goes here. If you iniate combat with one of the guards they will all come after you. When they do, simply walk back to the city with the men after you and once you get in, the men will stop near the entrance. After this you may simply sneak in and free the slaves without killing a soul.

As long as you are not seen by the patrolling guard, you're able to simply try each gate until all the slaves are free and simply walk out.