I was looking for a similar solution
New members are signing up to the forum, I would like them to automatically be on the monthly newsletter list. I can delete the list and recreate it, but surely that not the correct way to do it.
I cant find this info in the manual, so thought I'd see if the question had been asked before.
I've looked at the default subscribed when editing the mailing list. Then brought up a list of subscribed users, but the latest few users to sign up to the forum are not on the subscribed list.