Don't worry. The best parallel I can draw to understand communities would be sub-reddits. Some you may have the request permission to post on some while other are completely open. I wouldn't be surprise to have private communities popping up too. But by default, I believe people will have their communities open as it is the best way to get engagement.

The advertising would work exactly like in the screenshot I took of the webdev subreddit.

I personally don't see the causal link between scammers and advertising opportunities, so I can't really comment on how to discourage those behavior.