add has 24 distinct synonyms spread across 5 of its 7 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. Its 2 recorded opposites are take away, subtract.
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.
make an addition (to); join or combine or unite with others; increase the quality, quantity, size or scope of
“We added two students to that dorm room”
constitute an addition
“This paper will add to her reputation”
a condition (mostly in boys) characterized by behavioral and learning disorders
Synonyms, antonyms and the broader and narrower relations for add come from Princeton University’s WordNet. Full sources and licences.