If I want to enter another person on a registration form only if needed, how do I show or hide the s

I am not sure I fully understand the question, but the easiest way would be just have couple additional fields on the form that can be left empty or filled in.

Or you can try to use skip logic rules to hide/show additional fields, here is link to help http://help.adobe.com/en_US/formscentral/using/WSd789abd336388b1642c7fce012da94afe4f-8000. html#WS0faeceb8c23f5d60-774f03e3131ddc76d7d-8000

Or you can try to use page skip logic, here is brief overview of this feature http://forums.adobe.com/thread/869959

Thanks

Roman