I think it's less formal than the 'invite only'. Maybe, more of a 'welcome mat'.
Having said that, let me invite you to be a part of a group I've started, Relevant Review. Tricia Protzman (level 3) is on board and we are seeking a strong writer like you to come along with us. We want to add a Level 1 and Level 2 to help along.
Our group will stay close to its name. We want our reviews to be relevant to the challenge grading system. It's our desire to do more than the pat on the back that often is the comment. We're wanting meaty feedback.
I have always followed your writing as I think you're one of the best on FW. I also know you are true to your style.
Join us and let's work on helping a couple younger writers as we learn and hone our skills from one another.