You can add your own custom fields in the sign up form within two easy steps.
Firstly, you have to enter your own field. To do this, go to Settings, General and click Additional client-related fields in the Custom fields settings section.
You can now add your own field e.g. address.
Secondly, go to Synchronization with WWW and next to form click Edit > Fields completed by clients. At the very bottom you will see the fields you have added before, you can now add them to the sign up form.