The recipient of the hint #reunite has to guess the identity of the person who sent the hint. It is never simply revealed in this case. To add to the confusion ;) there is another type of hint that only reveals the identity and not the hashtag.
Is there anything to prevent guessing dozens of people until you guess the right one?
Nope! Beyond a lot of clicking :)
OP suggested that they might add a cost per guess.
That sounds like both a good way to both monetize and rate limit spam guessing. I'm not seeing where OP suggested this idea, so I'm crediting you! Thanks. Or maybe I'm OP?