Creating a user group?

[RedGTiVR6]

I need to figure out a way to create a user group.

Here's what I need to have happen:

Users with 0 post count are in one user group. Those users cannot post in the classifieds section. Once they have at least one post under their belt, they can post in the classifieds.

We're getting a lot of folks who register for our forums just so they can sell stuff. They have no intention of actually participating in the community.

How would I set this up? I've been told that I could do it with user groups, but I can't figure this out.

Perhaps there is an article on this that I've missed?

Any help would be appreciated!