>How to subclass group classes, so they will contain my classes as members? Ex: How to make MyFormSet, so it will containe MyForm class, not basic Form class?

1. Add a FormSet you your class library
2. Change the form controls toolbar so that it display your class library instead of the VFP default.
3. Drag as many form into the formset that you need.
4. Click on the original form (not based on your class, then click the "class" menu then "remove form"

Formsets are not as useful as they once were though. I haven't used one in a few years.