If you don't want guests to be able to view user profiles and the memberslist: Change the permission for Userground Unregistered / Not logged in.
If you do want guests to be able to view profiles but not the memberlist:
In memberlist.php
FIND
PHP Code:
// permissions check
if (!($permissions['forumpermissions'] & CANVIEW) OR !($permissions['genericpermissions'] & CANVIEWMEMBERS))
{
REPLACE that with
PHP Code:
// permissions check
if (!$bbuserinfo['userid'] OR !($permissions['forumpermissions'] & CANVIEW) OR !($permissions['genericpermissions'] & CANVIEWMEMBERS))
{