If you look at the conf_global.php file in your FTP, there is a line member_group that defines the default registration member group (usually group ID 3). Effectively what you want to do is
Set conf_global.php member_group to be the group you want all newly registered users in (in your example "NEW").
Set up a group promotion rule so that after 10 posts, the user is moved into the better member group ("Members" in your example)
That's it.
