An administrator can enable certain experimental features in Settings > Site administration > Development > Experimental > Experimental settings.
An experimental feature is defined as a feature which require additional testing and bug-fixing.
Note for sites upgrading from earlier versions of Moodle:
The experimental setting 'Group members only' was removed in Moodle 2.8. The group restriction option in Restrict access now provides this functionality. Any group members only instances are automatically converted to group restrictions as part of the site upgrade process.