join vs league together

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

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

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

join (noun)	league together (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)	league together (verb)
make contact or come together	attach oneself to a group
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