Hi! I'm Christian, baptised and everything, so this question doesn't apply to me specifcally.
I was wondering why you don't allow non-Christians to participate in bible studies. I can understand why you wouldn't want one to lead one, but wouldn't it be more evangelistic to allow them to ask questions and try to lead them to the Lord?
Please read and meditate upon 1 Cor. 2:1-16 for your answer. To discern spiritual things one needs the indwelling Holy Spirit.