ZotPad has a closed beta, to which you can apply at http://www.zotpad.com/beta.The number of beta testers is limited to around 70 because of Apple's limitation on the number of test devices per developer. People who have previous experience in beta testing software and who have been active at github and Zotero forums are prioritized.
Before applying as a beta tester, you should really think why you want to do that. Being a member of the beta test team does not mean that you have access to robust implementation of new features earlier than others. Instead it means that you will be working with an unfinished software and helping to make it better. Because the number of test device registration is limited, it is important that these registrations are used by people who are really committed to testing and troubleshooting unfinished software.
You can read more about beta testing here.