$sql="SELECT *
FROM user
WHERE username='$username'
AND password='". md5($password) ."'
AND (usergroupid=5 or usergroupid=6 or usergroupid=7 or usergroupid=8)";
or
$sql="SELECT *
FROM user
WHERE username='$username'
AND password='". md5($password) ."'
AND usergroupid IN (5,6,7,8)";