Custom widget not rendered within field group
OK. I finally found the problem based on the example code here.
The problem was that I constantly used "form" instead of element. Once I had replaced that the rendering was correct.

$form['b2share_fieldset']

instead of 
$element['b2share_fieldset']

However, the huge downside is that now I don't know how to access the field information as $form_state isn't working anymore.

EDIT: After applying some minor changes in the code I can confirm that this approach works. The fully functional code can be seen on github.