The subjonctif passé makes clear that the party needs to have arrived before the start of the ceremony. Whenever you want to emphasize the sequence of events, i.e., that one needs to have completed before the other one starts, you use the subjonctif passé.
Just to add to what Chris said that I totally agree with, I think this is a problem with how in English compound tenses are often avoided flouting the rules of 'concordance des temps' because they sound a bit weird. This is often the case with the use of the pluperfect.
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