There is a feature that will autopromote a person to a group based on number of posts.
Check:
Control Panel » Group Management
look at the user and preview row
to the right will be a column called post count.
If you have say a 1 in user post count. as soon as that user posts one time he will be promoted to user group.
Also highest is 1 lowest is 0 literally.
even though the note says 0 disabled and 1 enabled
