league together vs join

league together is not a noun while join is a noun.

Word	Noun	Adjective	Verb	Adverb
league together	No	No	Yes	No
join	Yes	No	Yes	No

As verbs, join is a hypernym of league together; that is, join is a word with a broader meaning than league together:

Other hypernyms of league together include fall in, get together.

league together (noun)	join (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

league together (verb)	join (verb)
attach oneself to a group	make contact or come together
	cause to become joined or linked
	become part of; become a member of a group or organization
	come into the company of
	be or become joined or united or linked