The Rokkit Rangerz are a small Ork tribe that formed a part of Warlord Gorgutz's WAAAGH!. It is common for Ork tribes to become part of other, larger Ork tribes if their Warboss is killed by another Ork leader. Warlord Gorgutz used this method to quickly build a massive Ork invasion on three separate occasions; first on Lorn V, then Kronus and finally a massive assault was launched by these Greenskins upon the Kaurava System.
Most other large scale Ork invasions (like the wars for Armageddon) have been formed in this way. One tribe must simply slay another tribe's leader, and the most likely outcome is that the tribes will merge and eventually begin to form a WAAAGH!.