WordNet (r) 3.0 (2006):

sign up
    v 1: engage by written agreement; "They signed two new pitchers
         for the next season" [syn: sign, contract, sign on,
         sign up]
    2: join a club, an activity, etc. with the intention to join or
       participate, "Sign up for yoga classes"

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "sign up":
   affiliate, affiliate with, alternate, associate, bespeak, book,
   brief, call up, combine, come into, commandeer, conscript, cover,
   creep in, detach, detach for service, do a hitch, do a stint,
   do a tour, do time, draft, employ, engage, enlist, enroll, enter,
   fill in for, get into, go into, have a go, have tenure, hire,
   hold office, impress, inaugurate, induct, initiate, install,
   instate, invest, join, join up, keep a watch, league with, levy,
   list, mobilize, muster, muster in, ordain, preengage, press, raise,
   re-up, recruit, reenlist, relieve, reserve, retain, serve time,
   sign on, sign up for, sneak in, spell, spell off, summon,
   take into employment, take on, take out membership, take turns,
   take up membership, team up with, team with, time off