sign up vs hire

sign up is not a noun while hire is a noun.

Word	Noun	Adjective	Verb	Adverb
sign up	No	No	Yes	No
hire	Yes	No	Yes	No

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

Other hypernyms of sign up include employ, engage.

sign up (noun)	hire (noun)
	the act of hiring something or someone
	a newly hired employee

sign up (verb)	hire (verb)
join a club, an activity, etc. with the intention to join or participate	engage for service under a term of contract
engage by written agreement	engage or hire for work
	hold under a lease or rental agreement; of goods and services