Although I have heard some Italians maintain that fra means "between" (two elements) and tra stands for "among" (more than two) e.g. "he's standing between John and Mary" and "we're among friends." To add to the confusion, some maintain the opposite.
But to cite an expert (although language is not like mathematics, and there is not the same level of precision), namely Giorgio di Rienzo in his column on language matters in Corriere della Sera, the two are interchangeable, apart perhaps for optional considerations of sound, such as avoiding alliteration. Although one hears "fra la folla" (among the crowd) all the time.
Remember that tra and fra can also mean "in" to denote a future time lapse: il treno arriva tra un'ora, the train will arrive in an hour.
