The Space Admin controls are found in the Space Tools menu at the bottom of the left sidebar.
If you're the owner of a space or you've been given Space Admin permissions by the owner, you'll see menu options to control user access and permissions, change the look and feel of the space, and modify other settings.
Space Administrators are the first level of support for granting access to their wiki space. They are empowered and have the capability (and are better equipped to know the levels of permissions that would be appropriate) to manage users into their Wiki Space. Space Admins have full access to the tools that enables them to do that. Wiki Admins support them if they run into issues.