You can specify a Target Audience in a Webpart, for it to only be visible to e.g. members of a certain AD group.

Have you ever wonderd yourself why on earth the new group does not appear in the list and is not valid? Well, I did :-(

The reason is quite simle. Shortly after the userimport from your AD or another LDAP source there is another import started. This one imports groups and memberships. Only after this step the groups will show up in the "Target Audiences" field in your Webpart!

But remember. This Target Audience is only for displaying a Webpart to certain users. It has nothing to do with access rights to items!

You can start the import in your SharedServices Provider http://urltossp/ssp/admin/_layouts/ProfMain.aspx