First of all I give a frowny face to everyone who suggested that a graduate student try to divvy up a $500 "young scientist best presentation award" with post-PhD personnel. At least in US academic culture, that borders on the ridiculous. Postdocs make about twice as much money as graduate students, and professors make at least three times as much money as graduate students. $500 may actually make some difference in a student's life: e.g. they could perhaps buy a new bed rather than sleeping on a futon with a broken plank (to take an example from my own graduate student days). If I was a coauthor on this paper I would have absolutely no expectation of a share of the $500. I wouldn't be offended by such an offer, but in my mind it would be a terrible mistake to put any of my student's prize money in my own pocket. (I gave @Stephan Kolassa's answer a downvote for the suggestion that the OP is compelled to do this or it will haunt him for the rest of his career. No way.)
I agree with Dan Romik's answer, which calls attention to the fact that the people who made the award are probably as familiar as we are with academic life and linguistic nuances: when they say the award is for the presentation and that it is awarded only to the presenter as a young scientist, surely they mean precisely that. If one agrees with the ethics of this then it seems to me that the only people the OP should contemplate splitting the award with are other graduate students who were involved in preparing the presentation.
He says that the prepared the majority of the presentation, which I find to be of some significance. In my much more individualized branch of academia I have never collaborated with anyone on a talk or conference presentation; even when I am describing joint work, the description of it is entirely my own. (Occasionally I have had the chance to watch a collaborator present on joint work with me, which can be very enlightening: their presentation is always different from mine; sometimes it is better, sometimes it is worse, and sometimes it is just different.) It could be (but might not be; I won't pretend to know) an assumption of those who gave the award that the OP singlehandedly prepared the presentation. If he got significant help from someone else, then that should have been clearly reflected in the presentation itself, and it might be worth raising that issue with the conference organizers. Here I would determine significant both by the total proportion of the presentation prepared and its role in essentially improving the presentation. Routine help should be paid back with gratitude and reciprocal routine help. Many professionals could feel embarrassed by the suggestion that routine help entitles them to a share of an academic prize.
I do want to admit that one could question the ethics of a prize which is given to an individual for work which is known to be collaborative. In my field (mathematics) awards given for conference presentations are very rare, and I remember that within the last year I read about a fairly prominent award given for a "best presentation" at a conference. I can't quite summon all the details now, but I do remember scouring CVs of the various parties involved to try to figure out whether the collaborators on the paper were getting short-changed for this. I found it a bit weird. However for a graduate student with eight coauthors to refuse a $500 prize on ethical grounds seems really impractical. If all parties agree that this is an improper recognition of an individual for a team effort, then it could be handled internally by not viewing the money as being individual money at all: it could be used only for expenditures by that group of coauthors, e.g. as funding for a different coauthor to speak on the work (or some continuation of it).
Finally, let me say that sometimes academics make mountains out of molehills, and I see a bit of that in the answers to this question (including mine!). If the OP takes the money, writes warm thanks to all of his collaborators, and tells his advisor to let him know if any further action is warranted on his part, I think he'll be fine.