sign up vs employ

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

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

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

Other hypernyms of sign up include engage, hire.

sign up (noun)	employ (noun)
	the state of being employed or having a job

sign up (verb)	employ (verb)
join a club, an activity, etc. with the intention to join or participate	put into service; make work or employ for a particular purpose or for its inherent or natural purpose
engage by written agreement	engage or hire for work