Thanks for pointing that out. It looks like that setting has been removed. Existing users are now automatically added to the group. This is probably the desired behaviour for most cases. One thing I noticed is that when saving the changes after a domain has been added to the setting, a modal window lets you know about the changes, but there is no obvious way to back out of the change:
I’ll update this topic’s first post to describe the new behaviour. The copy that we use for the “Automatic” setting also needs to be updated to make it clear that it affects all users on the site.