SEO communities on Skool range from large free groups to focused paid memberships. The biggest is Scalers with 1.9k members. 2 of the 10 are free, and paid memberships run from $17/month up to $79/month.
If you are starting out, the free groups are the low-risk way in, since you can judge the content, the people and the activity before paying anything. Paid memberships up to $79/month tend to add live coaching, structured programs and smaller, more accountable rooms. Weigh community size against how active a group is, and judge each price against what you get.
Also worth a look: The New Search, Rank Masters, SearchGAP Method. Or compare all 10 SEO communities by price and level.