The easiest way is o edit the template
Member List > member_list_show
Add this as the first line of the template
Code: 
<if test="showMlist:|:$this->memberData['member_group_id']==4">
add this as the last line of the template
Code: 
<else /><h1>Member List Not Available! </h1></if>
This allows only the admin to use the member list..