How can I make GrowlTunes start automatically when either Growl or OSX does?
Another, possibly quicker way, is to drag it into your dock (if not already there), then right-click it, go to Options>Open at Login. Then drag it out of your dock if you don't want it to stay in your dock.
Go to Preferences -> Accounts -> Select your account -> Select the "Login Items" button or tab (not sure what I would call it!) -> Then press the + sign to add a new start up item and select GrowlTunes.