User group admins are members of your account who can manage user groups (but they don't necessarily have to have any other admin rights).
External users and collaborators cannot be user group admins.
Group admins can:
Group admins don't necessarily become members of the user group they manage. If they are not in the user group, then when you share something with that user group, it is not also shared with the group admins (unless you add them to the group).
Account owners and account admins with the right to edit and delete users and collaborators can make users group admins. When you make someone an admin of a certain user group, they automatically have admin rights in all of its subgroups as well.
Account admins, due to their extended user rights, can manage any user group in the account even if they are not a group admin of that user group. But it's possible to restrict account admins from managing user groups using controlled admin permissions.
User group admins have an additional selector in the Users section of the Settings, it allows them to switch between all groups and the list of groups they manage.
To find the selector:
Click your profile image in the workspace's upper-right corner.
Select Settings from the dropdown.
Click Users in the left panel to open a list of all users and groups in the account.
Click the dropdown above the list of groups.
Select All groups to see the full list of groups in the account or Groups I manage to see groups where you're an admin.
The Groups I manage view doesn't display the hierarchical structure of groups and their subgroups. Instead, you'll see just a plain list.