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В гостях
in phrases with быть, сидеть, etc., e.g. быть в гостях to be in someone's home, to be visiting someone informally, to be at a small informal gathering

В гости
in...

гости is nominative plural.

в is never followed by nominative

в + Accusative (through, towards, at)

or

в + Prepositional (as, while, in)