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It's called "no-show" if a guest doesn't appear. Is it right to call the opposite "show"?
attending,existing & present are the opposite words used related to guests.
"No-show" is a person who said they would be there, but never arrived.
The opposite of a no-show is someone who said they would NOT be there but who showed up anyway.
In the context of an attendance list, show/no-show would be readable and has good parallelism
Required, but never shown