This article explains what type of access is permitted for Administrators to private groups.
Depending on the Administrator specific permission, Administrators may be able to access even Private Groups.
As you can read on the official documentation, there are several levels of permissions available for Administrators.
The only permission that allows people to see Private Groups is "Full Access".
Any administrator with "Full Access" will be able to see the Private Groups, edit them, and view the content published in them.
Jive recommends setting Manage System or Manage Community for Administrators that should not be able to access Private Groups.
This can be changed from Admin Console > Permissions > System Administration:
Find the group you want to change permissions for, and click Edit permissions, and select everything excluded "Full Access":
Make sure to select a group that does not include yourself (you don't want to lock yourself out of Full Admin), and click "Set Permissions".