Detecting all admins that are logged in

WordPress do not have a true concept of “logged in”. Although the labels all around use the term, in the code the closest you get is something that might be called “authenticated users”. The difference is that the authenticated user might be one that has his browser closed and therefor he is not “logged in”.

To do what you want you will first need to define what does it mean for you to be “logged in” and implement some logic to detect it. For example you might use ajax to update that a user is still connected and remove the user from the “logged in” list if it didn’t update for lets say 5 minutes.

It is not trivial but not very complex to implement.