join vs unite

join is a noun but unite is not a noun.

Word	Noun	Adjective	Verb	Adverb
join	Yes	No	Yes	No
unite	No	No	Yes	No

As verbs, unite is a hyponym of join; that is, unite is a word with a more specific, narrower meaning than join:

As verbs, join and unite are synonyms defined as:

Other synonyms of join include connect, link, link up.

join (noun)	unite (noun)
a set containing all and only the members of two or more given sets
the shape or manner in which things come together and a connection is made

join (verb)	unite (verb)
make contact or come together	become one
cause to become joined or linked	join or combine
become part of; become a member of a group or organization	act in concert or unite in a common purpose or belief
come into the company of	bring together for a common purpose or action or ideology or in a shared situation
be or become joined or united or linked	have or possess in combination
	be or become joined or united or linked