As an individual with a visible physical disability (some visual impairment) who has some autistic spectrum tendencies (though not diagnosed) I am an outcast most places I go. I am trying to improve myself to be less of one but here are some tips for groups that want to improve their diversity.
Designate a point person to welcome and include people who are on the periphery. This can be someone who has social skills and some connection to the core members of the group. If you are trying to reach marginalized groups it helps if this person is a member of said group but that is not imperative. If one person in the group is making those on the periphery feel welcome that will help these folks to open up and build connections with some of the other members. The analogy is like plastic wrap, the first tear is the hardest.
Follow up online with new members. This may mean talking with them on Slack or Discord or a similar service. If you built rapport with them they can start speaking more freely. Also if they show you their writing or projects pay attention. People who are marginalized tend to self sabotage by clamming up in groups they feel they don’t belong. This has happened to me frequently. So keep in mind the people you see may have a lot more to contribute to the group than they appear to due to this.
Be aware of structural barriers. I once saw a faith based conference where people were grousing about how the group attending wasn’t diverse enough. It cost $150. Lots of marginalized groups are also low income and often don’t have cars. They may not be able to get to or afford what you have to offer. Mitigating this can be difficult and inconvenient but you must decide how much you are willing to sacrifice in order to become more diverse.
Be kind. The world is becoming an increasingly cruel place. This hurts the people without power the most because they can’t counter bad actors. The more of an asshole you are the more racist/sexist/homophobic/ablest actions you are going to perform even if you yourself harbor no ill will toward minorities. The problem with kindness and virtue is that it’s “caught not taught” so even if you know the right thing to do in your head, you usually end up aping what you see in your surroundings.