automatic integration with Membership plugin

I've created my first newsletter, but I see that some people are still registering using WP, though I took the meta box out. I want to send the newsletter ONLY to members who register using the Membership registration, so that it goes out to paying groups, each group is a different subscription plan. What should I do to set this up and avoid people registering in the WP for free and still receiving the newsletter that is only for members?