Thank you, Jürgen, for this post.
I was struggling to get the normal search in List&Search type to work with groupx fields.
Reason: setting up groupx I followed the tutorial where it says to set storage for the fields in the groupx content type to "None".
After following the instructions here in this thread I almost got it working.
One important piece of information was missing, though:
You have to set storage for your groupx field also to custom/article/introtext[groupxfieldname].
Otherwise search won't work.