One thing to note on this. They will only be added to that group when they make a new post. What I mean by that, is this:

Say, you set it so they need 100 posts to be in that group. At that time I have 125 posts. I won't be a member of that group until I make a new post somewhere.

So it checks to see when they post if they need to be added to a group, it doesn't automatically add users to that group when you change the number in the control panel.