The groups in the Bookmarks window are initially named for importing the svnX 1.x prefs.
They are fully customisable and arbitrary in number, order, content & name.
By default new WC bookmarks are added to the group containing the currently selected item (or the first group if no selection).
However, you can simply drag your (or any) bookmark into the desired group.
[Also drag bookmarks & groups to reorder them. Press option while dragging to duplicate them.]
Hope that helps.
Regards,
CHRIS