The feature to allow membership after a certain amount of posts is a great idea, but not relevant for me. though I did use it for one forum.
I'm thinking more along the lines of password entry.
It may make it clearer if I explain my board.
It is a study board for fellow students, but I would like others, such as those thinking of enrolling on the course or those simply interested in the subject, to be able to visit and communicate to students.
So I want visitors but don't want them having access to specific study forums.
I will include an explanation in my welcome e-mail or such as you suggest and will keep my ear to the ground of any upgrades you make.
While I'm here (good grief!) my personal pic avator shows up here but not on my own board. huh!?