The index grows from the networks' own seller files, so most sites arrive on their own. If one you care about is missing, here is how to queue it — and what has to be true for it to appear.
Email the domain to [email protected] with the subject “Submit”. One domain per line, no other formalities. We will queue it for the next crawl: its ads.txt is read, its homepage is fetched, and it is checked against every tracked network's seller file.
A site earns a page when there is something verifiable to say about it: a tracked network's seller file lists it, or its own ads.txt names a tracked network. A site with neither has no relationship for us to record, and adding it would just be a page with nothing on it. Submitting cannot make a network's file say something it doesn't — that part is between the site and its network, which is rather the point of the method.
Sites also stay off the index while they have nothing to compare against — no niche classification yet, or a homepage that could not be read. Those usually resolve within a few crawl cycles.