Must / should / nice method
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Must: you would not marry without them (close family, intimate friends)
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Should: you want them there, but can explain a smaller format
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Nice: you see them once a year, invite out of courtesy
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Rule: invite established couples together, not one spouse alone
Common cases
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Coworkers: only close team, or none for a very intimate wedding
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Children: global decision (all invited or adults only), stated on the invitation
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+1: for guests without a partner you know, not automatic for everyone