Use username password on different subdomain
Yes,there is no problem using  drupal user table, you just need implement you site and  connect to data base that Drupal install on it and then just need use user table ,
only one note remain that the password  saved with md5 method in  user table.
I thing is not complex, the summery of answer is connect to db use uid(or name) and password field of users table .

as sample code :

&lt;?php

$host="localhost"; // Host name
$username=""; // Mysql username
$password=""; // Mysql password
$db_name="drupaldb"; // Database name
$tbl_name="users"; // Table name

// Connect to server and select databse.
mysql_connect("$host", "$username", "$password")or die("cannot connect");
mysql_select_db("$db_name")or die("cannot select DB");

// username and password sent from form
$myusername=$_POST['myusername'];
$mypassword=$_POST['mypassword'];

// To protect MySQL injection (more detail about MySQL injection)
$myusername = stripslashes($myusername);
$mypassword = stripslashes($mypassword);
$myusername = mysql_real_escape_string($myusername);
$mypassword = md5(mysql_real_escape_string($mypassword));

$sql="SELECT * FROM $tbl_name WHERE name='$myusername' and pass='$mypassword'";
$result=mysql_query($sql);

// Mysql_num_row is counting table row
$count=mysql_num_rows($result);

// If result matched $myusername and $mypassword, table row must be 1 row

if($count==1){
// want to want
echo "congratulation ,you login";

}
else {
echo "Wrong Username or Password";
}
?&gt;