Luis,
Setting a group for a K2 user isn't a part of JFBConnect. However, I'm pretty sure it would take a 1 line code change to hard-code it so that all new users through Facebook / JFBConnect would be automatically added to the group you want. If you subscribe and want to do what you say above, we can gladly help guide you in making the change. It shouldn't be too difficult.
Hope that answers your question, but if you need anything else, just let us know!
Thanks,
Alex