employ vs sign up

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

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

As verbs, sign up is a hyponym of employ; that is, sign up is a word with a more specific, narrower meaning than employ:

Other hyponyms of employ include featherbed, fill, engage, ship, contract, sign, sign on, rat, farm out, job, subcontract.

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

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