Sign up Definition 1 of 3Pro Tip 1/3Temporary Commitment Use 'sign up' when you're joining something short-term or with a clear end date. She signed up for a six-week pottery class. Pro Tip 2/3Optional Participation 'Sign up' usually indicates voluntary participation, not mandatory. You should sign up for the debate club if you're interested. Pro Tip 3/3Action Required Implying action, 'sign up' often involves filling out a form or online registration. To attend the workshop, sign up on our website.