Welcome to Meta 
I can reproduce. I think it’s by design, but I agree that it’s confusing. And there should be an explanation.
If everyone is allowed to read, post, and create topics, then adding another group doesn’t change permissions.
What do you want to achieve with the group you add?
Maybe the text would already help. But as I reported before, it is only shown when the default locale is English, so “everyone” is in English