The Volley option for the Hunter ranger’s 11th-level Multiattack feature says:
You can use your action to make a ranged attack against any number of creatures within 10 feet of a point you can see within your weapon’s range. You must have ammunition for each target, as normal, and you make a separate attack roll for each target.
The hail of thorns spell description (PHB, p. 249) says:
The next time you hit a creature with a ranged weapon attack before the spell ends, this spell creates a rain of thorns that sprouts from your ranged weapon or ammunition. In addition to the normal effect of the attack, the target of the attack and each creature within 5 feet of it must make a Dexterity saving throw. A creature takes 1d10 piercing damage on a failed save, or half as much damage on a successful one.
Is it possible to use Volley with hail of thorns?
I believe that when the character hits the first creature with Volley, hail of thorns is triggered.