[Paul M]

Try this ;

Create a plugin linked to the register_addmember_process hook.

Put this code in it (adjust the two variables to suit).

PHP Code:

$spamgroup = 4;
$checkfield = 'field6';

if ($vbulletin->GPC['username'] == $vbulletin->GPC['userfield'][$checkfield])
{
    $userdata->set('usergroupid', $spamgroup);
} 


Note, this is untested.