how we can add new field in join form ??

Hi,

 I have dolphin version 6.1.6. I am making  a social networking website. I want to some change in field or want to add some new field in Join Form or join.php page. But I am unable to find such code where I can add a new field in join form. Please suggest me how I can do this or where I can find such code.

Thanks

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