Have command blocks show all online Admins on your Minecraft server
Just a way I figured out to show any online admins on a server.
It works using a score called 'admin', when set up to a clock it will repeatedly delete the admin score (which removes the sidebar list), re-add the admin score, set it to display in the sidebar and then update the admin score of any online admins.
This doesn't have to be used for admins though, there can be any condition for the listed players, you could have it list players with over 100 kills or players that are over level 20 etc.
Essentially this is a list that removes players from the display when they go offline.