They Come And Go
This add-on will allow you to get a message posted to the FIFO message system (the messages that appear near the top of your screen) or to a dedicated window whenever a friend or guild member goes on- or offline.
You can toggle the messages for when friends come online, when they go offline and when guild members come online and when when they go offline.
If you have a guild member also added as a friend it will only show the friend message and not the guild message as well.
You can also set a bunch of options for the dedicated window to make it fit your liking.