If "Sie" is being used for "You" (singular, formal), then it isn't clear what "all" would mean here. If you want "You (pl) all arrived", I think it would need to be something along the lines of "Ihr seid alle gekommen". Now if only someone who knew what they were talking about would comment, I'd be happy!
kommen is an intransitive verb of movement, so it has sein as the auxiliary verb in compound tenses rather than haben. So in this sentence sind could be translated as "have" if you were using the English present perfect translation of the German Perfekt tense (as in "They have all come") or you could just translate sind gekommen as the English simple past tense "came" as the given correct solution does.