Roles for users in WordPress

Hi,

I’m planning to integrate some forum with WP205.
Actually, in the forum the people can do new registration, but some of belong to an special group, with special permissions.
The problem I have is I don’t know how to have an special group in WP. Because WP have just 4 roles for users.
In fact, all what I need is some users of WP can access to a certain category. This could be an special role or not.

But because everybody can register in my WP installation, I don’t know if it’s possible to do all that.

Thank for your help.