Unlike other enemies in the game, warbands will travel far and fast, bringing conflict when you least expect it. These enemies ride on both wargs and horses, wielding terrible instruments of war.
The studio says that there are no "dibs" when fighting warbands. Any player who attacks a warband will get credit for the kill, even if not grouped. Turbine says that it created this open tapping system to encourage cooperation instead of competition.
[Source: Turbine press release]