Although anyone can download the app [Direct Link], Cameos requires access to be granted once it's downloaded. Celebrities who gain access can browse the Internet's "top questions" asked about them, choose which ones they want to answer, and record a video of their response.
Cameos will push celebrities with new questions and searches to ensure that their Google-related content "stays recent and timely." With the app, high-profile users can respond directly to their fans' biggest questions and leave out any potential misinformation provided from other websites sourced from a Google search.
Once a question is answered, Google says that the response videos get posted "right to Google" so that users can see the videos when they search for the celebrity, or similar questions about the person. The videos will also appear on each celebrity's feed in the main Google app.