Volunteer in this case means that it’s not really a paid position with guaranteed hours, not that it’s something anyone from the public can volunteer for. The PyPI access needed for this, and the decisions involved, requires an extremely high level of trust.
The best way to get involved is somewhat adjacent to it. Become familiar with Warehouse (the code that runs PyPI), then start triaging issues, making high quality PRs, answering questions, etc. The more work you can help with there, the more time the people who do have trusted access will have for this work. It’s not an easy or quick thing to do, you need to stick with it for a long time. But it’s something anyone can do, and establishing long term commitment and trust through that will go a long way to maybe being able to handle these requests.