sign up - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Enlist in an organization; also, register or subscribe to something. For example, He signed up for four years in the navy, or Are you planning to sign up for that pottery class? [Early 1900s]