Erkenbrand: Considering he has a summon ability right now, that makes me view him as the unit supporter captain.
Courage of the Westfold: All nearby friendly units gain +30% movement speed and +25% damage and armor, while next to Erkenbrand or his 2 summoned units. Description: "Erkenbrand is a mighty leader of men, granting courage in times of doubt. All units near to Erkenbrand or his summoned men of the Westfold gain +25% armor and damage, and 30% movement speed"
Voice of Helm Hammerhand: Erkenbrand and his summoned units stun nearby enemy units with terror for 10 seconds and reduce their armor by 50% for 30 seconds. Description: "Erkenbrand shouts in a mighty voice reminiscent of the legendary Helm Hammerhand, followed eagerly by his men, stunning nearby enemy units with terror for 10 seconds and reducing their armor by 50% for 30 seconds"
Elfhelm: His current ability replaces dead soldiers in a battalion, so naturally he is the unit healer captain. I would also suggest renaming his current ability, considering it doesn't actually heal units, but only replaces 1 dead unit in each battalion.
Elfhelms Encouragement: All nearby heroes get healed by a small amount, similarly to Athelas. Description: "Elfhelm provides encouragement to nearby friendly heroes, helping them to forget their wounds and fight on. All nearby friendly heroes will be moderately healed"
Marshal's Orders: Refreshes the ability timers of all nearby units, including other Captains of Rohan, but not heroes. Description: "Elfhelm orders nearby men of Rohan to fight to their last breath, urging them to greater efforts in combat. The ability timers of all nearby units, including other Captains from the Assembly Point, will be fully recharged. Does not affect heroes"
Grimbold: His current ability doesn't ever seem to work for me, but if it did work the effect places him as a mass slayer.
Wrath of the Eastfold: Grimbolds unit will deal double damage, ignore enemy armor and knock back enemy units they hit for 30 seconds. Knockback and armor ignore doesn't work against enemy heroes. Description: "Grimbold urges his Oathmen to fight harder, filling their hearts with fury. All units in Grimbolds battalion will deal double damage, ignore enemy armor, and knock enemy units back when attacking them. Does not affect enemy heroes"
Muster of the Eastfold: Targeted stables will be able to recruit Rohirrim of the Eastfold at half price, and train them instantly, for 1 minute. Description: "Grimbold is in command of mustering the warriors of the Eastfold. The targeted stables will be able to recruit Rohirrim of the Eastfold at half of their current price, and train them instantly, for 1 minute"
Please feel free to post your own ideas and suggestions for new abilities for each Captain from the Assembly Point