Here's a simple tip to persuade people to join any type of group.
Highlight what the group isn't.
This latches upon two key aspects of psychology.
The first is people understand any object better when it's contrasted with another. I don't know if Car A is good by itself, but I can tell if it looks better than Car B. If it does, Car A seems good.
The second is we tend to see things in good/bad dualities. We see most choices as a choice between something good and something bad.
You can influence members to see joining the community as a choice about who they are and what values they hold (or want to hold) Are they group A or group B type of people?
If they're not Group B, they must be Group A. This is known as self-categorisation in social identity theory.
We are far more likely to categorise as part of a group when a contrasting group is made salient. If they're not one of them, they want to be one of you.
So highlight who the community isn't for. Don't be patronising, simply highlight the type of people who wouldn’t be a good fit for the community. It might be people that are new to the field, people that have specific beliefs, people that don't match a prototypical member.
You don't even have to make group B sound terrible, just make sure Group A is slightly better.
We spend too much time explaining who the community is for (usually everyone, sadly) and too little time explaining who it's not for.