The rule of thumb that I've always used is that if you have to login to view, then Google can't get in to view it.
So if it's a password protected forum (or directory), Google can't index the pages as it doesn't have the password.
Now there's always exceptions to the rule, but that's the general rule that we use when seeing out links of influence.
It looks like the site isn't blocking anything, so all the pages are getting indexed.
Looks like events created on this page are indexed too https://nextdoor.com/events/476278/
This could be a good community to get established in, which would provide seo benefit and a community benefit. I'll stress not to use it just for SEO purposes... that could lead to abuse if not careful. Use it to drive more of a presence in the community, and think of the SEO as an added benefit. Market the business correctly, and use it if your customers are there. Links don't create revenue, but strategic marketing could.