All Mac's Registries are free to join, it costs you nothing.
No personal owner data are ever divulged or published unless the owner specifically wishes to have it published in one of the registries. Your data are kept on file and if someone wishes to contact you, I will forward any email to you. It is then up to you if you wish to respond to them.
To see Mac's Family of Registries, click the Mac's Chevelle/Camaro/Nova/HHR/SSR Registries link at the top of any page.