Available for group members only

From MoodleDocs

Note: The information on this page only applies to sites upgrading from earlier versions of Moodle.

Available for group members only was an experimental feature in earlier versions of Moodle and was removed in Moodle 2.8. The group restriction option in the Restrict access settings now provides this functionality. Any group members only instances are automatically converted to group restrictions as part of the site upgrade process.