We all hate to see this message when we want to login to the server :-
“The terminal server has exceeded the maximum number of allowed connections.”

All you need is the PsLoggedOn utility from sysInternals (now owned by Microsoft). Another reason we should all love Mark Russinovich and SysInternal tools.
Just follow the steps :-

1. Download PSTools from here.
2. Extract the zip file to a folder.
3. Open command prompt and navigate to the extracted folder.
4. Run the following command. Keep the \\ and replace the MachineName with the fully qualified name of the machine where you want to see who are currently logged on.

5.   You will see who are logged on to that machine.

psloggedon \\MachineName 

This saves you sending emails to a group to find out who are logged into the server. Rather – because you know who are logged in, you would ping/email those specific users, asking them to log off.