As a side-project, I started a community called "Local SEO Community", centered around local SEO specifically (as opposed to SEO more broadly), and we recently crossed over 400 members.
My plan is to keep it free-for-life for every who joins prior to reaching 1000 members. After that, I plan on charging new members ~$50/year to join.
I think this scarcity has helped grow the community so far. Members have invited others because they know the free spots won't last long.
It is hosted on Circle, which has been a great experience so far. I have given them a few suggestions here and there and they are super fast about replying and implementing the suggestion in many cases.
Feel free to check it out here: Local SEO Community
If you have any feedback for how it could be improved, let me know!