I'm trying to think of the best way to answer this. I think you might be right for the wrong reason. I checked DL's tips and the chart does indeed show among as an acceptable translation; it is in fact in bold which I can only assume it is also one of the more common transliterations (DL does a terrible job explaining their tips and what certain things mean and explaining what they are trying to convey, but I digress).
Here's why I think you are wrong: I don't think the number in context for voi should determine the translation. You can have three or more people ("group") form a circle around a person/people and that person/people would be between them. Conversely, if voi stood for two people ("couple") a person/people could be walking or standing around the couple and thus be among the couple.
That's why I think your reasoning is wrong, however, I do not think you are wrong in what this sentence could mean.
If we were to consider the direct (Ei) and indirect object (voi): Ei could be another pronoun (Eu, Noi, e.g.) and the sentence would still work, in either context (between/among). If we consider voi, we know this to be plural so între's translation has to make sense. So we need a word that compares the plurals and between/among still holds. If we changed the indirect object to the singular "you": tu then the context can change again. "They are between you(s)" does not make as much sense, although you might at least argue it could make sense metaphorically: "They are between you" as in "They are between [hiring] you [and not hiring you for the job]". If we meant it as "They are among you(s)" then this can make perfect sense. I think of a science fiction movie scenario where aliens infiltrate and live among humans on earth and nobody can tell who they are, and in a dramatic fashion the camera goes to the scientist that everyone thinks is crazy and he tries to warn them "They are [living] among you!" (although that sentence works for plural "you" just as much).
Here's why I think you are right: @languagepotato had a good (and unanswered) question as well. Does this sentence work literally and/or metaphorically. And, as you have pointed out, what is the context of this sentence. In this case "They are among you" could be a perfectly acceptable answer and I would have reported it to DL as a correct answer.
TL;DR: I am not considering the quantity of voi as I don't think the number of people is imperative to the translation. However, your answer still makes sense and appears to be a correct translation (at least in English, perhaps there is a rule in Romanian where între can only be "between" for humans and is "among" for inanimate objects??), therefore your translation should be accepted.