join vs conjoin

join is a noun but conjoin is not a noun.

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

As verbs, join and conjoin are synonyms defined as:

join (noun)	conjoin (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)	conjoin (verb)
make contact or come together	make contact or come together
cause to become joined or linked	take in marriage; married
become part of; become a member of a group or organization
come into the company of
be or become joined or united or linked