Comment by epistasis

Most of these groups are centered around a neighborhood, or a school, or a church. For anything school related, people are very suspicious of outsiders trying to join. Churches and neighborhood groups might be more open, I suspect, but still gotta get somebody who lives there or goes to the church to vouch for you.

But the worst case for an outsider joining is not very bad; they get to see what's going on, but the entire point of the endeavor is to bring everything to light and make everything more visible. And if an outsider joins and starts providing bad information or is a bad actor, typical moderation efforts are pretty easy.