All of those are hard to put a specific limit on. Especially when doing native material.
Any native material, even meant for children, will include grammar from every level, since they naturally know it. We can’t pin point it any more than “absolute beginner of reading native material”.
Minimum needed to join is also hard to say, since people use different methods and sources to learn.
All we can do is provide example pages and let people try it for themselves.