I've noticed in this video (around 12:45) that the guy explaining stuff about the game points to tags on the back or sides of the cars and says that this shows what faction they are in, like World of Warcraft.
So this must mean as well as joining a crew you can also be part of a bigger group, maybe something like the clubs in TDU only bigger hopefully.
This is probably where the rival crew thing fits into the game, going up against other factions.