Synonyms for Sign on:

n.

confirm, appoint, recruit, employ, retain, take on, hire, engage. DO, sit in on, involve, join in, participate, take part (in something), enter into, get in on, engage in. endorse, witness, sign away, John Hancock, signature, countersign, sign on the dotted line. jobless, job seeker, bloodbath, retired, employment agency, social exclusion, poverty trap, redundant, out of work. sign on (noun)
fee, sign, sign up, contract.

v.

draft (verb)
recruit, sign up.
employ (verb)
retain, hire, contract, take on, engage.
engage (verb)
appoint, employ.
enlist (verb)
enter into.
social (verb)
sign up, contract, sign.

Other synonyms:

hire, countersign, jobless, John Hancock, sign away, social exclusion, out of work, job seeker, retain, employment agency. signature, employ, retired. redundant, endorse, recruit. take on, confirm, engage. witness. Other relevant words:
employ, recruit, engage, retain, hire, appoint, take on.