enlist vs sign up for

enlist

verb
  • To recruit the aid or membership of others. 

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

  • To secure; to obtain. 

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

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

sign up for

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