Riders is the ingame name for the HQ of the Gale Motorcycle Club, the gang you will meet in Area 3. To access this area, take the pathway to Stone Village, and you should find the entrance in the cave.
When you reach this area, you will find that there is nothing to do but interact with a single man wearing a motorcycle mask, with many more around him. However, if you had previously rested at the area 3 camp and had a member of your party kidnapped, you will find that the entire gang has been killed, save for one member, Rex Thunderstorm, who is now a Joy Mutant. Upon Rex's defeat, you will find a pile of flesh to interact with, showing the text "Is this...?"
This message likely implies that the meat pile on the ground is the remains of your kidnapped party member, which is further supported by the fact that you never again see them.