The command /friends add/remove [playername] will provide notifactions when your friends sign on and off. You can add players from other servers by prefixing the server name and a period before the player name (e.g. emerald.playername).
From the orginzation info window you can select the friends tab.
Here the add and delete buttons make it easier for you to manage friends. The monitor button adds a monitor to your hud.