I've added a sub-groups feature to the groups widget.
If a group contains sub-group(s), they will displayed in the same way that individuals are currently displayed - comma, separated list in their own section. I decided not to just dump the sub-group members in to the list of individuals in the parent group because the sub-groups are frequently (or at least sometimes) included in the parent group as a group (e.g. 'sons of Thestius' are mentioned as a group when included in the Calydonian Boar Hunt).
Clicking on a sub-group name will display information about the sub-group.