IndexNow claims to distribute the pings to other participating search engines[1], and to participate they need to have "noticeable presence in at least one market", so yea still seems to be somewhat exclusive?

Eg, would Ahrefs or Semrush qualify to join the party?

1: https://www.indexnow.org/searchengines

Most search engine people don't call ahrefs or semrush search engines -- they're SEO tools.

Having to have existing market share as a search engine is going to really limit the number of participants.

There is a noticeable lack of information about how to enroll. I suspect it's more of a "if you need to ask you aren't big enough" type deal.

Or maybe just ask?

I've had staggering success with just ending emails to people, including businesses.

I agree, you can get surprisingly far by just asking.

That said, I can't find any contact information on the IndexNow website.

From the announcement blog (https://blog.cloudflare.com/crawler-hints-how-cloudflare-is-...):

This is an exciting problem to solve, and we look forward to working with others that want to help the Internet be more efficient and performant while reducing needless energy consumption. We plan on having more news to share on this front soon. If you operate a bot that relies on content freshness and are interested in working with us on this project, please email crawlerhints@cloudflare.com.

Perhaps I'm missing something but Crawler Hints seems to be completely separate from IndexNow — it's just a CloudFlare product that implements it.