>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...