Details were up in the air for how mounted group combat would be handled, but it is something Turbine is interested in pursuing.
Faq:
17. Can I group in mounted combat?
Sure. Nothing stops you from grouping and it may be a very wise move to coordinate an attack on a Warband with other players.
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What exactly can you tell us about mounted combat in a group setting?
I'd thought war-bands were basically planned to be a form of mounted-group combat....
Obviously Turbine is going to wait a while before doing instanced-mounted combat style stuff, but surely by now they know how mounted-group combat is supposed to work...right?
Going by the 1 trailer they have out so far that shows some in-game action it looks like it might just be 1 or 2 hits and the npc dies. Does not look like the "normal" warg rider npcs will be hard to kill. How this translates to the roaming war bands I have no clue.
*note that this is just my personal speculation*
If it is just 1 or 2 hits and the npc dies I do not see how much fun this could be nor how this could qualify as actual combat.
Raids and big group content might be the only real good part of mounted combat for a lot of people.
*once again note that this is just my personal speculation*