Great minds think alike (or fools seldom differ?), since I was musing about an altered version of War Cry that would work on Spectral Hyenas or other allies with DEX-based melee attacks.
This is what I came up with as a first draft:
Hunting Cry. As a bonus action, the gnoll beast shaman can utter a horrible howl, choosing up to two allied creatures within 60 feet [or 30 like the spell?] who can hear or see the shaman to form a hunting pack with. Allied creatures must be conscious to join a hunting pack, and leave the pack if knocked unconscious. This pack targets one creature chosen as prey by the shaman; this may be a creature within 150 feet who the shaman can see or smell, or a creature whose tracks the shaman can see or smell within 10 feet. These tracks may be up to 24 hours old, but at the DM's discretion favorable or unfavorable circumstances can increase or reduce this time limit (e.g. rain or hard and impermeable ground are unfavorable, snowy ground or blundering prey that leave an obvious trail are favorable). The shaman can also choose prey using the scent of objects a creature wore, such as clothes or footwear.
Hunting cry is an Innate Spellcasting ability that requires concentration. The beast shaman can use hunting cry when it already has an hunting cry active, using the bonus action to either change the hunting pack's prey creature (restrictions as above) or choose three creatures for the pack, up to a maximum of nine (the shaman plus eight allies).
The hunting cry lasts for 1 hour or until the shaman loses concentration. If the prey drops to 0 hit points, the shaman has until the end of its turn to make another hunting cry to choose a replacement prey creature for the pack; failing to do so ends the hunting cry. If a dispel magic spell targets a pack member or the prey creature, the target immediately stops being part of the hunt, this ends the hunting cry if the shaman is the creature dispelled. If the prey is dispelled, the shaman has until the end of its turn to use hunting cry to choose new prey (which may be the same as the old prey).
When a creature joins a hunting cry pack (which includes the shaman), they gain one hunting die, which the pack member must spend on the first melee attack it makes against the prey, adding 2 (1d4) to either its attack roll or its damage roll. The hunter rolls to hit first: if the roll to hit fails, it adds the hunting die to try turning the miss into a hit; if the roll to hit succeeds, it adds the hunting die to damage. The hunting die replenishes if the shaman chooses the creature with another hunting cry (it can choose itself). The shaman cannot give a pack member two or more hunting dice, if can only replace a die the hunter has spent.
In addition, as long as the hunting cry is active, all pack members gain the following trait:
Predatory Senses. The creature has advantage on any Wisdom (Perception) or Wisdom (Survival) check it makes to find the chosen prey.
