The term 'signed up' can be used to describe a variety of actions. 'ENTERED' is a 7-letter word that means to come or go into a place. 'Enlisted' is an 8-letter word that means to join the military or other organization. 'ENROLLED' is also an 8-letter word that means to register for a course or program. 'Registered' is a 10-letter word that means to sign up officially for something. Lastly, 'Recruited' is a 9-letter word that means to find suitable people and encourage them to join an organization. All of these words can be used to describe the action of signing up for something.