sign up for (something)

When you "sign up for" an event or activity, it means that you fill out a form that shows that you want to join. For example:

I signed up for an aerobics class at the gym.

We decided to sign up this year for the Walk Against Cancer.

You can also "sign up" someone else (usually a child) for an activity:

Maybe we should sign Gavin up for violin classes. 

This phrase appears in these lessons: