Shoutbox

Using the Event Notifications option of Plus can do this for you

Click Preferences>Customize!>Event Notifications

Then click "Add"

Select the contacts you wish to add the notification for by checking the boxes next to their name (although you cannot automatically add an entire group, you can still manually add all of the people in it and just update the event notification if you add/delete someone to that group on messenger)

Then click "Next"

Then select the "User has signed in" option and press "Next"

Select how you would like the notification to display (Toast/Sound/Both) and then press "Next" to finish

If you only want to be notified when these contacts come online then you will also need to disable Windows Live Messenger's own alerts by going to:

Tools>Options>Alerts and Sounds

and unchecking the option "Display alerts when contacts come online"