join vs link up

join is a noun but link up is not a noun.

Word	Noun	Adjective	Verb	Adverb
join	Yes	No	Yes	No
link up	No	No	Yes	No

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

Other hypernyms of join include connect, link, tie.

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

Other hypernyms of join include connect, link, tie.

As verbs, join and link up are synonyms defined as:

Other synonyms of join include connect, link, unite.

join (noun)	link up (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)	link up (verb)
make contact or come together	make a logical or causal connection
cause to become joined or linked	connect, fasten, or put together two or more pieces
become part of; become a member of a group or organization	be or become joined or united or linked
come into the company of
be or become joined or united or linked