According to the D&D 5th edition Player's Handbook, the description of the Help action in combat states (page 192; bolding made by me):
You can aid a friendly creature in attacking a creature within 5 feet of you. You feint, distract the target, or in some other way team up to make your ally’s attack more effective. If your ally attacks the target before your next turn, the first attack roll is made with advantage.
And in the Sprite's statistics in D&D 5th edition Monster Manual (page 283):
Invisibility. The sprite magically turns invisible until it attacks or casts a spell, or until its concentration ends (as if concentrating on a spell).
So my assumption is: if I tell my sprite to help me with casting my eldritch blast by:
- Flying invisibly around my enemy, and pointing out good spots to hit to me, helping me to aim.
- Spilling small amount of sand from above the enemy to distract him.
- Or something similar,
Then I gain advantage on my first attack with it (so at 5th level, I'll get advantage on my first missile, but not the second). And as the sprite is neither attacking nor casting, he remains invisible (but of course can still be sensed by some creatures, or heard by them).
My only concern is: Is such help strong enough to provide me with advantage?
I know that probably this question is resolved by "talk to your GM", because rules are not strict, but still I would like to hear your opinions on this topic!
Best Answer
Since the question on whether or not a sprite can remain invisible and use its help action has already been answered, I'll jump to the one bit that hasn't been answered so far:
Yes you get advantage
The Help action itself is quite remarkable (and underused in my experience): you give up your attack - and thus your ability to do damage yourself - in order to give one of your allies advantage on its next attack on one specific creature you choose.
You give up your Sprite's damage to the foe and in return you get a greater chance to hit, and thus have a bigger chance of doing damage yourself.
The way your sprite's Help action plays out is mostly flavour. What is does, however, is determined by the Help action's definition, which you've already found.
My answer to your question would be: Yes the Sprite would grant you advantage.
But...
You have already stated you're aware of the fact that creatures could sense the fact that your sprite is standing/flying next to them, when you wrote: "(but of course can still be sensed by some creatures, or heard by them)"
Keep in mind that your Sprite has to be within five feet of the enemy in order to take the Help action and can still be attacked while invisible, although this means the attacker will do so with disadvantage, due to the way the Invisible condition is described:
If your sprite were to stay in the enemy's reach, this means they could attack it on their turn, but with disadvantage. Your Sprite can, however, fly out of the enemy's reach without provoking an attack of opportunity, assuming it has any movement left.
RAW-wise, an attack of opportunity can only be made against a creature the enemy can see, which gives your Sprite the possibility to aid and move out of the enemy's reach without taking any damage and without requiring the owl's "Flyby" feature.
Edit: I found out this question has already been answered, for more elaborate explanations, go here.