membergroupids IN (18) doesnt work - why?

[kafi]

Can anybody help me with this querry? I have users that are in 2 membergroupids. That is 12 and 18.
When I write condition membergroupids = 12 it works fine and it pull out ousers that are in 12 membergroupids (aka additional usergroup). I guess this works only because 12 is recorderd as first id in table... 12,18...
But when i try conditional membergroupids = 18; or I try membergroupids IN (18) neither doesnt work.

Any idea why? How to accomplish this querry? Is it because there are two "IN" in the querry???

This is what I try and doesnt work for me:

PHP Code:

$query4 = $vbulletin->db->query("SELECT userid FROM user WHERE userid IN ($user) and membergroupids IN (18)"); 
while ($row4 = $vbulletin->db->fetch_array($query4)) {
     echo $row4['userid'].";";
     echo "<br />";
} 


Milion thanks!!!