Use 'sign up' when you're joining something short-term or with a clear end date.

Temporary Commitment

'Sign up' usually indicates voluntary participation, not mandatory.

Optional Participation

Implying action, 'sign up' often involves filling out a form or online registration.

Use 'sign up' for services or organizations when planning to stay for a while.

Indicates the beginning of membership or service usage.

You often need an email address to sign up, especially for online services.

Emphasizes a choice to engage in public, social, or volunteer activities.

'Sign up' implies that you agree with what the activity stands for.

Shows you're taking the initiative to get involved or contribute.