Hi mariuszstach,

Try this:

// note: double quotes around the outside,
// single quotes surrounding each username.
$usernames = "'some_guy23','another1','person3'";

$query = "SELECT username "
. " FROM #__session WHERE username in ($usernames) "
. " AND guest = 0 "
. " AND client_id = 0 ;" ;
$db->setQuery( $query );
$found = $db->loadAssoc();
if (is_array($found)) return 101; /* found him. customise to your own module number. */
return 102; /* didn't find him. Customise to your own module number. */