Hi Don:
Thanks for the reply/suggestion. I tried it, and while it seems to get the job done in terms of assigning the new registered users to the K2 group registered, it seems to be making wholesale/major changes to the default K2 user screen. K2 users are no longer on that screen, and managing those is impossible.
I was thinking more in line with the register pop-up triggering a K2 registration, as opposed to a Joomla user registration - perhaps that would get the job done?
To see what I mean, please take a look at your demo for World News II: after a successful FB login, the user is Joomla registered; under K2 users, the Joomla user also appears, but not belonging to any group.