The first option includes secret beta testing links. This was the method commonly employed by Google+ beta tests, although there’s now no requirement to join a community – you just need to have the link. Additionally, developers can place a hard cap on the number of testers.
The second option allows for closed betas using email addresses. Developers will be able to let individual users through, or they have the option to upload a CSV file with a list of email addresses that can have access to the beta test.
The former methods will continue to work, as well – and that’s good, because we like options!