Shared subset of users in a multi-site instance Can I have a shared subset of users in a multi-site instance? In other words, I have the same list of administrators across all sites. Can these administrators be admins for all sites through a shared database rather than producing the same list of admins for each site? 


I am aware that each site in a multi-site instance has its own database, which contains content and users specific to said site. 
I am aware that content and users can be shared by configuring a shared database. However, I only want to share administrators among sites and keep content and other users with site-specific roles and permissions specific to its respective site.
As you mentioned yourself you can share users across multiple site but that are all users. So you need an external 'database' for your admin users that magically exists for all sites.

For this you could use https://www.drupal.org/project/ldap which could contain you administrators.

Normal users can login as normal aka without use the LDAP directory.

Note LDAP can work with MS Active Directory too.