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The whole family gathers at dinner.
Literal
Dinner [at-には] family everyone [subject-が] assemble.
そろう ('to be complete, to assemble, to come together') is distinct from 集まる ('to gather') in that そろう implies the completeness of a set — all expected members being present. 家族全員 ('the whole family,' literally 'family + all members') emphasizes totality. A gently warm description of a nightly family custom.