When activating the securlet, Selective Scan is chosen. In the selective scan screen under Define Scan Policy, as a groups or users is typed into the field, it shows matches that can be selected. But some groups or users do not appear. The Start Scan and Add Rule buttons may be greyed out.
The selective scan screen does not show all values in the drop down even if they are valid options. Type in the full group or user name, then hit Enter, and it will add your value to the scan even if the value does not appear in the drop down.
NOTE: This is case sensitive. Check the case that is used in Active Directory