ragteks nuns will allow you to send a welcome pm, start a welcome thread, post to an existing welcome thread. you can do any or all of those and define the user account you wish to do the posting/sending. its pretty cool. http://ragtek.org/xenforo/threads/ragtek-new-user-notification-system.129/
Any other addons that do this other than ragtek's? Doesn't seem that he's supporting his addon as the threads are all closed and per Blueprint4Love's experience.
It would be nice to be able to use the notice system to send scheduled messages based on usergroups and actions. I think that would be a great feature to add!