join has 13 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 one recorded opposite is disjoin.
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.
become part of; become a member of a group or organization
“He joined the Communist Party as a young man”
the shape or manner in which things come together and a connection is made
1 of the 7 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 join.
Synonyms, antonyms and the broader and narrower relations for join come from Princeton University’s WordNet. Full sources and licences.
a set containing all and only the members of two or more given sets
“let C be the union of the sets A and B”