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!