Go to the agenda that you want sign up to.

You can sign up to a role that is marked "Open" and has a blue sign up button.

Click on the button.

The page will then show your name.

The example given is a linked role (the same role appears more than once on the agenda), and the name appears in all places automatically.

The sign up button has changed to "Release". Use this if you need to remove yourself from the role.

Depending on how many roles you still have in the meeting, you may see the following:

This lets the Toastmaster of the meeting (and any others who get notified) keep track of how many members will be at the meeting.