If you've given it a try, you'd know that you can't create a Group Focus inside another Group or a Repeating Group. The Group Focus always stays at the page level and it cannot have a parent group.
There is a neat workaround to this limitation. We simply create a Reusable element.
Creating the Reusable element
The design of the Reusable element can be anything as we'd like.
We then create a Group Focus in the Reusable element and set up triggers to show/hide this Group Focus.
If you'd notice, the Group Focus still does not have any parent group apart from the Reusable element itself. However, the Reusable element can have a parent group.
As a result, we can place this Reusable element inside a Repeating group cell. This allows us to have a Group Focus inside a Repeating group.