You can restrict access to an article by going to the Manage Articles admin menu; selecting the article you wish to restrict; and then clicking the "restrict" link for that article. This allows you to restrict the article based on whether a user is in a certain membership category or user group.

By the way, it's a similar process if you wish to restrict a Forum to a particular user group -- go to the Manage Forums admin menu; click the edit link for the forum you wish to restrict; and then select from the "Restrict to Group" drop-down menu the user group you wish to restrict the forum to.