enlist vs sign up

enlist

verb
  • To secure; to obtain. 

  • To recruit the aid or membership of others. 

  • To enter on a list; to enroll; to register. 

  • To voluntarily join a cause or organization, especially military service. 

noun
  • One who is enlisted, usually in a military service. 

sign up

verb
  • To agree to purchase some good or service. 

  • To add one's own name to the list of people who are participating in something (including amateur, but not professional, teams) 

  • To add a name to the list of people who are participating in something. 

How often have the words enlist and sign up occurred in a corpus of books? (source: Google Ngram Viewer )