In the case of the clusters, you can find them by product (accesories, apparel, jewelry, decor, etc) or by color. These clusters are curated by the the website owner, but you can also create your own once you register to the site.
To apply to the site you simply write to Rachel (who is a great help, by the way) on the contact tab.
You can also visit the blog
Here you can see one of their curated clusters where 2 of my earrings were included.