Apologies for the delayed response. We never like posts that go this long unanswered, and this one just slipped through.
Unfortunately, there isn't a way to have the redirect open in a new window. The redirect that happens after the user hits "Send Invitations" is performed by Facebook itself. JFBConnect tells Facebook where to redirect the user, but there's no way we can let them know how to do it.
Hope that helps at least explain, and best of luck to you!