If you connect a node N to a group G, then the contents of G are not considered successors of N and hence that content is not displayed in the successors view.
If your successors view is "Hierarchy Aware" (which is the default and may be changed from the successor view's context menu), it displays parent nodes in addition to selected nodes and successors of selected nodes.
(Moreover, in "Hierarchy Aware" mode connections into closed group nodes are resolved.)