>Then how can you have a community that is welcoming to people who are not part of the ingroup?

Being welcoming to every random person is by definition not a community, it's a free-for-all mess.

A community means communal interests and values, it's in the name. And to guard those you can't just be accepting everyone without vetoing them. That's how it turns to a shit of spammers and trolls and people who want to hijack it and don't share the original cause/spirit. Has happened to forum after forum...

You'd have to be brutal about culling, uninviting and removing anyone who doesn't look like a good fit.

Or have a two-stage process: run very public, very open events that anyone can sign up to an attend. And then invite specific people that you meet at those events that look like a good fit for your community to your private, community-only event.

> for whom no one can vouch

Spot the fed