Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
join (v.)
can take any of several prepositions in Standard English: to or with, as in Our committee was joined to [with] theirs; in, as in The whole room joined in the chorus of booing; even I joined in. Commentators sometimes object to join up and join together as redundant, but join up is a Standard idiom meaning “to enlist in the armed forces,” as in He joined up as soon as the school term ended, and join together is also Standard, simply emphatic, and very familiar at that, as in the traditional marriage ceremony words from Matthew 19:6,… what therefore God hath joined together …). See REDUNDANT.
