Only platform administrators can see all the groups (whatever their privacy is) from the groups admin page which can be found from the admin menu > "Manage groups".
From this list, all the groups are displayed (except deleted groups).
2 warnings can be shown:
when a group has no administrator, a blue warning shows that a new admin has to be selected
when a group hasn't got any new activity since 6 months, a red warning is displayed
From this list, an administrator can:
change the privacy, title, description of a group
manage members of the group (make them group administrators or kick them)
add users: from this action, the administrator can look for users (find all the members of another groups or all the members with a specific skill...), select all the found users and add them to the groups
make himself group administrator