The way two warring guilds would not group together is cause they would do damage to each pother and that would do the group no good. Imagin this... Say Gimli is in the Dwarven Hammer guild and Gandoff is in the Mages of Magi guild. The two guilds are warring but both, Gimli and Gandolff are friends with Stryder, Legolas, and Frodo and the three of them ask them to join their hunting group. The group goes out and are attacked by a swarm of Uruh-ki and Gimli being the tank goes out to the front. Gandolf must now do croud controle and goes to mezmerize the attacking Uruh-ki. In doing so, since his and Gimli\'s guild are warring and they have active PvP for each other, Gimli gets mezmerized and can no longer fight. The Uruh-ki then pass his motionless form and attack the rest of the group. Styder is tough but being a ranger he doesnt have the armor that the warrior dwarf has and cant take the pounding from the attacking Uruh-ki. Frodo by no means can even dream about taking damage so he has to slip on the one ring and vanish. This leaves the robe wearing mage, the frail elf, and the light armored ranger to now take on this group of Uruh-ki which they have no hopes of surviving without a tank.
So there you have it. Two warring guild members would be restricted from a hunting group not by any game rule but simply cause the group leader has to think of the entire group\'s survival. Having two members that are set active PvP against each other means one can inadvertantly hurt the other and that is added group damage that is totaly not acceptable.