Current RAW (or at least my reading on it)
Alternatively, 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.
This wording specifies a singular ally. It doesn't say "the next ally that attacks"1 or "your allies' attack" or "If one of your allies" or anything similar, which strongly implies it affects only one ally, which is chosen by you.
Additionally, note that distracting the enemy is one possible way to help, but there are others. In particular, note this sentence. Again, this implies that interacting with the enemy creature itself is not even needed.
or in some other way team up
Conclusion: You help your ally (one specific ally) in attacking a creature.
Rules as Intended is different, though
1 According to John Caroll's answer, it actually does say that in the 2017 DM Screen. I would expect some clarification or errata on the PHB from that. The "in some other way team up" is also excluded and distracting is the only way to help in combat.
Or you distract one creature within 5 feet of you, and the next attack roll that an ally of yours makes against that creature has advantage.
This wording is consistent with the answer by Crawford in twitter
If you use the attack-aiding option in Help, the next ally who attacks the target gets the benefit. #DnD
Note that this answer is inconsistent with this other answer from him.
Q: Can a Mastermind Rogue use the Help action as both a normal Action & a Bonus Action in the same turn to assist 2 different PCs in combat?
Since the advantages would simply "stack up" in the same attack and be lost2. Note that the tweet about assisting two different PCs is more recent than the tweet directly saying "the next ally attacking gets the advantage".
If the intended is that the next person attacking gets the advantage, an errata should be published for that, IMO.
2 As noted by V2Blast, this is not true if two different enemies are chosen - but the wording in the question is, at least, unclear.