HubNugget isn't something one can join. Each week, the HubPages staff selects three categories of Hub topics. Then they nominate six Hubs within each of those topics that were written by new Hubbers. The HubPages community and others (friends and family members of the nominees, for instance) vote on the article within each category that they like the best.
The top three authors in each category receive a HubNugget Accolade. My winning Hub was written less than three weeks ago.