attendant has 14 distinct synonyms spread across 4 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
a person who is present and participates in a meeting
“he was a regular attender at department meetings”
an event or situation that happens at the same time as or in connection with another
occurring with or following as a consequence
“an excessive growth of bureaucracy, with attendant problems”
1 of the 5 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for attendant.
Synonyms, antonyms and the broader and narrower relations for attendant come from Princeton University’s WordNet. Full sources and licences.