On a multisession host be able to select which user you would like to chat with

If you launch a session to a multisession system such as RDS, you can switch between the users logged into the session and chat directly with that user.

From the messages area, before you join a session, I would like to be able to have the ability to send a message to a specific logged-in user or all users currently logged into the system.

For example, this would allow us to message a specific user on a multisession server requesting permissions to connect to their session before connecting. It would also allow us to send a message to all users currently logged in.