'sign up' definitions:

Definition of 'sign up'

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

Synonyms of 'sign up'

From: Moby Thesaurus