The Sin City Bikers are motorcyclists, They are more than likely to be members of a corrupt motorcycle gang or club.
In Sin City: A Dame To Kill For, after Nancy and Marv had talked, they head out to kill Senator Roark. After exiting Club Pecos, they see the bikers approaching. They have intentions are to possibly, steal from the club-goers or taking over the club. Marv approach one of the bikers, gave him a headbud whilst taking his gun, then he shoots the remaining bikers. Marv allowed Nancy to kill one of the bikers which she did. She then got a hold of their crossbow and then they used their motorcycles to get to Roark's house.