Wow.. 1 year without reply... @Klas, hopefully you have better answer...
If you're working with Intro, Content, OR List.. you probaby like this manual:
https://www.seblod.com/resources/tutorials/calling-field-x-and-group-x-content-in-an-override
BUT, i see that you're referencing on Site/Admin Form, than it's little bit different story... One of my approach is altering the output instead of tweaking the process (Desclaimer, i don't fully understand about how groupX render process in technical perspective).
So, here's an example of mine:
I would like a certain user group get their select list disabled... my approach is to eliminate/erase all-non selected option. Code goes something like this:
$content=$cck->renderField( 'termin_bayar' );
$pattern = '/<option value="[0-9]*">.*<\/option>/i';
$replacement = '';
if (key($user->groups)==10){
echo preg_replace($pattern, $replacement, $content);
}else{
echo $content
}
$user object is automatically load within seblod template override, no need to initial this jUser object