How to create extra fields
Open a customer and click the "More Fields" button, shown here:
two empty boxes will appear:
fill the boxes in and press save.
Once the "Extra Field" has been created, go to Settings > Extra Fields and click on it to change the settings for this particular field (e.g. change it from "text" to a "date" field, or make the extra field visible from the main customer listing).
Some extra fields will be made available on the customer signup form. Check the signup form to see if the extra field is available (Settings > Signup)