WordNet records no synonym and no antonym for standee. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
someone who stands in a place where one might otherwise sit (as a spectator who uses standing room in a theater or a passenger on a crowded bus or train)
“the allowed number of standees is posted”
a lifesize cardboard cutout (usually of a celebrity)
“he had his picture taken with a standee of the president”
Synonyms, antonyms and the broader and narrower relations for standee come from Princeton University’s WordNet. Full sources and licences.