Now first of all, the previous movies wouldn't really clear that up either, they simply had completely different people and groups on their own, not related to the story or characters of this movie at all.
The movie doesn't explicitly explain the whole political situation, but that post-apocalyptic world it depicts is split into various smaller groups and clans which pretty much all fight for their own survival and interests. It's just a big mad carnival of insanity. However, some of them still have some kind of alliances with each other while others might have wars and others simply don't give a damn (or don't even know of each other) and just judge each encounter on its own.
But you did indeed mix some things up here. The War Boys led by Immortan Joe seemed to have some kind of alliance (be it merely trade-based or even deeper) with the guys from the Bullet Farm and from Gas Town, which is apparent from the fact that Furiosa is supposed to drive that water/guzzoline/mother's milk tanker to those places and trade that stuff for other goods.
Immortan Joe: Once again, we send off my War Rig to bring back guzzoline from Gas Town and bullets from the Bullet Farm!
War Boy: We got vehicles from the Citadel. They're firing flares. They want reinforcements from Gas Town......and the Bullet Farm.
When she then leaves the road to Gas Town heading west, she leaves that groups' territory and enters the territory of some other clan (the "Buzzards" with their spiked cars), who are either pissed at being hurt in their "territorial rights" or simply want the stuff that's in the tanker. The War Boys even help her fight against them (since she still carries a valuable good, the mothers, even if she "stole" them). She later also meets other guys that only have their personal interests in mind, like the motorcycle gang living in the canyon (the "Rocky Riders") with whom she has a deal to seal off the canyon in exchange for the fuel but who then go after her when they feel betrayed.
Observer: Your produce ain't going to Gas Town. She's gone off-road into hostile territory.
Furiosa: I made a deal up ahead. Safe passage. I don't know if it's still any good.
But the people that assist Immortan Joe's War Boys and follow their convoy after joining him at the entrance of the canyon are only the people from Gas Town (led by this fat guy with the nose thingy) and the Bullet Farm (led by this shooting-mad guy who later loses his eyesight). Only they help Immortan Joe in his cause because of their whatever-natured alliance, but even they remark at various points that they don't really see the use of such an exhaustive and resource-draining chase for what simply are "family problems".
Rocky Rider: You said: "A few vehicles in pursuit. Maybe." We count three War Parties!
